P2-type Na 0.67 Mn 0.72 Ni 0.14 Co 0.14 O 2 with K + doping as new high rate performance cathode material for sodium-ion batteries

Regulating Anionic Redox via Mg Substitution in Mn-Rich Layered Oxide Cathodes Enabling High Electrochemical Stability for Sodium-Ion Batteries

With the limited resources and high cost of lithium-ion batteries (LIBs) and the ever-increasing market demands, sodium-ion batteries (SIBs) gain much interest due to their economical sustainability, and similar chemistry and manufacturing processes to LIBs. As cathodes play a vital role in determining the energy density of SIBs, Mn-based layered oxides are promising cathodes due to their low cost, environmental friendliness, and high theoretical capacity. However, the main challenge is structural instability upon cycling at high voltage. Herein, Mg is introduced into the P2-type Na0.62 Ni0.25 Mn0.75 O2 cathode to enhance electrochemical stability. By combining electrochemical testing and material characterizations, it is found that substituting 10 mol% Mg can effectively alleviate the P2-O2 phase transition, Jahn-Teller distortion, and irreversible oxygen redox. Moreover, structural integrity is greatly improved. These lead to enhanced electrochemical performances. With the optimized sample, a remarkable capacity retention of 92% in the half cell after 100 cycles and 95% in the full cell after 170 cycles can be achieved. Altogether, this work provides an alternative way to stabilize P2-type Mn-based layer oxide cathodes, which in turn, put forward the development of this material for the next-generation SIBs.

Stabilized O3-Type Layered Sodium Oxides with Enhanced Rate Performance and Cycling Stability by Dual-Site Ti4+ /K+ Substitution

High-capacity O3-type layered sodium oxides are considered one of the most promising cathode materials for the next generation of Na-ion batteries (NIBs). However, these cathodes usually suffer from low high-rate capacity and poor cycling stability due to structure deformation, native air sensitivity, and interfacial side reactions. Herein, a multi-site substituted strategy is employed to enhance the stability of O3-type NaNi0.5 Mn0.5 O2 . Simulations indicate that the Ti substitution decreases the charge density of Ni ions and improves the antioxidative capability of the material. In addition, the synergistic effect of K+ and Ti4+ significantly reduces the formation energy of Na+ vacancy and delivers an ultra-low lattice strain during the repeated Na+ extraction/insertion. In situ characterizations verify that the complicated phase transformation is mitigated during the charge/discharge process, resulting in greatly improved structure stability. The co-substituted cathode delivers a high-rate capacity of 97 mAh g-1 at 5 C and excellent capacity retention of 81% after 400 cycles at 0.5 C. The full cell paired with commercial hard carbon anode also exhibits high capacity and long cycling life. This dual-ion substitution strategy will provide a universal approach for the new rational design of high-capacity cathode materials for NIBs.

Zinc-Doping Strategy on P2-Type Mn-Based Layered Oxide Cathode for High-Performance Potassium-ion Batteries

Mn-based layered oxide is extensively investigated as a promising cathode material for potassium-ion batteries due to its high theoretical capacity and natural abundance of manganese. However, the Jahn-Teller distortion caused by high-spin Mn3+ (t2g 3 eg 1 ) destabilizes the host structure and reduces the cycling stability. Here, K0.02 Na0.55 Mn0.70 Ni0.25 Zn0.05 O2 (denoted as KNMNO-Z) is reported to inhibit the Jahn-Teller effect and reduce the irreversible phase transition. Through the implementation of a Zn-doping strategy, higher Mn valence is achieved in the KNMNO-Z electrode, resulting in a reduction of Mn3+ amount and subsequently leading to an improvement in cyclic stability. Specifically, after 1000 cycles, a high retention rate of 97% is observed. Density functional theory calculations reveals that low-valence Zn2+ ions substituting the transition metal position of Mn regulated the electronic structure around the MnO bonding, thereby alleviating the anisotropic coupling between oxidized O2- and Mn4+ and improving the structural stability. K0.02 Na0.55 Mn0.70 Ni0.25 Zn0.05 O2 provided an initial discharge capacity of 57 mAh g-1 at 100 mA g-1 and a decay rate of only 0.003% per cycle, indicating that the Zn-doped strategy is effective for developing high-performance Mn-based layered oxide cathode materials in PIBs.

Revealing the role of dopants in mitigating degradation phenomena in sodium-ion layered cathodes

Prevention of the degradation of sodium-based layered cathode materials is the key to developing high-performance and high-stability sodium-ion batteries. In this study, the working mechanism of Mg and Ti dopants in mitigating degradation was investigated through the use of first-principles calculations. More specifically, the effects of each dopant in suppressing the phase transition, lattice expansion and shrinkage, and possible oxygen generation during repeated charging and discharging processes were validated. The results showed that the pristine structure exhibits irreversible O3-P3 phase transition after 75% desodiation, while doping with Mg or Ti effectively delays this transition. In addition, the change in lattice parameters as well as in the volume during desodiation was investigated. It was found that both dopants reduce the magnitude of structural change, which potentially improves the structural stability. Furthermore, introducing the dopants increases the thickness of the Na diffusion channel, possibly leading to an enhanced rate capability. Finally, the oxygen atomic charge variation during charging indicated that doping can enhance the oxygen stability by reducing the initial charge of oxygen as well as its increase during desodiation.

A novel Mn-based P2/tunnel/O3′ tri-phase composite cathode with enhanced sodium storage properties

A novel P2/tunnel/O3′ tri-phase composite Na_0.7Bi_0.01MnO₂ is developed for the first time by the Na⁺-site modification of Bi³⁺ for a high-performance cathode in SIBs.