Na⁺/Vacancy Disordered P2-Na_0.67Co_1–xTi_xO₂: High-Energy and High-Power Cathode Materials for Sodium Ion Batteries

Abstract

Although sodium ion batteries (NIBs) have gained wide interest, their poor energy density poses a serious challenge for their practical applications. Therefore, high-energy-density cathode materials are required for NIBs to enable the utilization of a large amount of reversible Na ions. This study presents a P2-type NaCoTiO (x < 0.2) cathode with an extended potential range higher than 4.4 V to present a high specific capacity of 166 mAh g. A group of P2-type cathodes containing various amounts of Ti is prepared using a facile synthetic method. These cathodes show different behaviors of the Na/vacancy ordering. NaCoO suffers severe capacity loss at high voltages due to irreversible structure changes causing serious polarization, while the Ti-substituted cathodes have long credible cycleability as well as high energy. In particular, NaCoTiO exhibits excellent capacity retention (115 mAh g) even after 100 cycles, whereas NaCoO exhibits negligible capacity retention (<10 mAh g) at 4.5 V cutoff conditions. NaCoTiO also exhibits outstanding rate capabilities of 108 mAh g at a current density of 1000 mA g (7.4 C). Increased sodium diffusion kinetics from mitigated Na/vacancy ordering, which allows high Na utilization, are investigated to find in detail the mechanism of the improvement by combining systematic analyses comprising TEM, in situ XRD, and electrochemical methods.

High Areal Capacity Na0.67CoO2 Bundle Array Cathode Tailored for High‐Performance Sodium‐Ion Batteries

“…In addition, cyclic voltammetry (CV) curves at a slow scan rate of 0.1 mV s −1 are provided in Figure e, in which cathodic peaks at 2.51, 2.79, 2.92, 3.11, 3.30, 3.76 V and anodic peaks at 3.63, 3.18, 3.03, 2.81, 2.62, 2.43, 2.19 V are identical to the CV results in Figure d. These voltage steps in charge‐discharge curves and multiple redox peaks in CV diagram indicates the phase transition in Na 0.67 CoO 2 , which is correlated to Na + /vacancy patterns based on Na + ions concentration and similar to most Na x CoO 2 metal oxides ,,,. Electrochemical impedance spectroscopy measurements (EIS) were imported to detect the electronic conductivity of Na 0.67 CoO 2 array cathode (Figure f), including its equivalent circuit diagram in the inset.…”

“…We noticed that the Na 0.67 CoO 2 bundle arrays well remained the architecture of Co(CO 3 ) 1‐x (OH) 2x ⋅nH 2 O medium even undergoing calcination treatment at 600 °C, which could be explained by the relatively mild calcination condition for fabricating Na 0.67 CoO 2 . In contrast to former reports around manufacturing Na x CoO 2 through high temperature (>800 °C) solid‐state method,, we put forward a state‐of‐the‐art “sodiation and calcination” strategy to tailor Na 0.67 CoO 2 only at 600 °C. In this case, the morphology deterioration of Na 0.67 CoO 2 in calcination process can be greatly alleviated benefited from the relatively mild condition.…”

“…The average coulombic efficiency is around 99.5 %, manifesting the highly reversible property of Na 0.67 CoO 2 array cathode. Even so, cycling performance of this cathode could be further optimized through following treatments such as graphene encapsulation, cationic doping and so on ,. Progressive variation of current rates were conducted for Na 0.67 CoO 2 array cathode to analyze the corresponding rate performance (Figure b).…”

“…The phase transitions readily results in structural deterioration and severe capacity fading. In this regard, suppressing phase transitions is considered to be an efficient way to enhance the corresponding cycling performance . Recently, it is validated analytically that engineering self‐supported arrays on conductive substrates with open structure is highly beneficial to transport of Na + ions and electrons …”

Layer‐Based Heterostructured Cathodes for Lithium‐Ion and Sodium‐Ion Batteries

“…This cathode material simultaneously realizes efficient utilization of the high capacity nickel‐rich core and high stability, which allow it to perform a capacity of 157 mAh g −1 and retain 80% of the capacity after 300 cycles. Performance modifications through compositional modification by metal ions including Mg 2+ , Zn 2+ , Cu 2+ , and Ti 4+ were also reported. For instance, Bruce and co‐workers performed systematic investigations on the influence of Mg substitution in Na 2/3 Mg 0.28 Mn 0.72 O 2 .…”