Lithium‐Substituted Tunnel/Spinel Heterostructured Cathode Material for High‐Performance Sodium‐Ion Batteries

Abstract

Sodium manganese oxides as promising cathode materials for sodium‐ion batteries (SIBs) have attracted interest owing to their abundant resources and potential low cost. However, their practical application is hindered due to the manganese disproportionation associated with Mn3+, resulting in rapid capacity decline and poor rate capability. Herein, a Li‐substituted, tunnel/spinel heterostructured cathode is successfully synthesized for addressing these limitations. The Li dopant acts as a pillar inhibiting unfavorable multiphase transformation, improving the structural reversibility, and sodium storage performance of the cathode. Meanwhile, the tunnel/spinel heterostructure provides 3D Na+ diffusion channels to effectively enhance the redox reaction kinetics. The optimized [Na0.396Li0.044][Mn0.97Li0.03]O2 composite delivers an excellent rate performance with a reversible capacity of 97.0 mA h g–1 at 15 C, corresponding to 82.5% of the capacity at 0.1 C, and a promising cycling stability over 1200 cycles with remarkable capacity retention of 81.0% at 10 C. Moreover, by combining with hard carbon anodes, the full cell demonstrates a high specific capacity and favorable cyclability. After 200 cycles, the cell provides 105.0 mA h g–1 at 1 C, demonstrating the potential of the cathode for practical applications. This strategy might apply to other sodium‐deficient cathode materials and inform their strategic design.

A Novel Pentanary Metal Oxide Cathode with P2/O3 Biphasic Structure for High‐Performance Sodium‐Ion Batteries

“…Because high‐spin Mn 3+ induces Jahn–Teller distortion, appropriate heteroatom substitution is expected to improve the structural stability and reduce the dissolution of TMO 2 layers. [ 9 ] The Mn 3s exchange splitting (Δ E 3s ) in NM, NMFM, and NMFML are 4.92, 4.58, and 4.49 eV, respectively, further confirming the increase in the average oxidation state of Mn induced by the co‐doping strategy (Figure S3g, Supporting Information). [ 32 ] It is expected, based on charge balance theory, that NMFM and NMFML are richer in surface oxygen vacancies than NM.…”

“…The direct imaging of the Li in NMFML by EDS is difficult because of its lightweight and weak electron‐beam‐scattering ability. [ 9 ] The above results indicate that NMFML consists of an intergrown P2/O3 biphasic structure rather than a simple mixture of P2 and O3 phases.…”

“…Because the presence of Fe and Mg dopants in the NMFM cathode disturbs the charge ordering and smooths the phase transformation process, the charge–discharge profiles of NMFM slope in the voltage range of 2.5–4.1 V. [ 14 ] The additional presence of Li + in an NMFML cathode not only improves its structural stability and phase transformation reversibility through strong Li−O bonding but also increases the discharge capacity it delivers owing to its higher proportion of O3 phase. [ 9 ] Notably, the voltage plateaus in the charge/discharge profiles of NMFML at ≈2.7 V are attributable to the phase transformation of O3 to P3 in the heterostructured cathode. [ 13 ] The Na and dopant contents have been optimized based on the comparative rate capability and cycling stability of the different co‐substituted samples (Figure S6, Supporting Information).…”

“…As shown in Figure 2e, the calculated D Na+ values of NMFML range from 10 −10 to 10 −11 cm 2 s −1 , which is almost twice that of NM. The Na + diffusivity of the cathodes can also be evaluated by measuring the slope of the plot of i P (peak current of the CV curve) versus the square root of v (scan rate), [ 9 ] as shown in Figure S14 (Supporting Information). The average D Na+ of NMFML is estimated to be 1.34 × 10 −10 cm 2 s −1 , which is much higher than that of NM and NMFM cathodes (Table S3, Supporting Information).…”

Strain Engineering by Local Chemistry Manipulation of Triphase Heterostructured Oxide Cathodes to Facilitate Phase Transitions for High‐Performance Sodium‐Ion Batteries

“…[23,24] Besides, according to current reports, the spinelrelated-type structure could stabilize the host cathode structures and simultaneously play a crucial role in boosting electron transport for coordination with the timely sodium ion (de)sodiation, which is due to the excellent intrinsic conductivity and the inherent high stability. [25] Therefore, many researchers focus on composite structures on account of multifunctional advantages, especially for multiphase structures. [26] In addition, cathodes with high operating voltages are essential for the development of high-energy-density SIBs, but the cycling stability of cathodes is still a pressing issue.…”