Rate Dependent Performance Related to Crystal Structure Evolution of Na_0.67Mn_0.8Mg_0.2O₂ in a Sodium-Ion Battery

Abstract

Sodium-ion batteries are considered as a favorable alternative to the widely used lithium-ion batteries for applications such as grid-scale energy storage. However, to meet the energy density and reliability that is necessary, electrodes that are structurally stable and well characterized during electrochemical cycling need to be developed. Here, we report on how the applied discharge current rate influences the structural evolution of Na 0.67 Mn 0.8 Mg 0.2 O 2 electrode materials. A combination of ex situ and in situ X-ray diffraction (XRD) data were used to probe the structural transitions at the discharged state and during charge/discharge. Ex situ data shows a two-phase electrode at the discharged state comprised of phases that adopt Cmcm and P6 3 /mmc symmetries at the 100 mA/g rate but a predominantly P6 3 /mmc electrode at 200 and 400 mA/g rates. In situ synchrotron XRD data at 100 mA/g shows a solely P6 3 /mmc electrode when 12 mA/g charge and 100 mA/g discharge is used even though ex situ XRD data shows the presence of both Cmcm and P6 3 / mmc phases. The in situ data allows the Na site occupancy evolution to be determined as well as the rate of lattice expansion and contraction. Electrochemically, lower applied discharge currents, e.g., 100 mA/g, produce better capacity than higher applied currents, e.g., 400 mA/g, and this is related in part to the quantity of the Cmcm phase that is formed near the discharged state during a two-phase reaction (via ex situ measurements), with lower rates producing more of this Cmcm phase. Thus, producing more Cmcm phase allows access to higher capacities while higher rates show a lower utilization of the cathode during discharge as less (if any) Cmcm phase is formed. Therefore, this work shows how structural transitions can depend on the electrochemically applied current which has significant ramifications on how sodium-ion batteries, and batteries in general, are analyzed for performance during operation. 50 cathode materials for Na-ion batteries due to their high 51 reversible capacity, and for M = Fe and Mn, cost and safety. 2−4 52 These materials share many common features with their Li-53 counterparts despite the larger size of Na. Sodium layered 54 oxides are typically identified using Delmas' notation 5 P2, P3, 55 O3, etc., where P and O indicate the Na sites (P = trigonal 56 prismatic and O = octahedral) and the number relates to the 57 transition metal layers within the unit cell. From this 58 classification, numerous studies have been reported for P2-59 type Na y MO 2 materials demonstrating their higher capacities, 60 diffusion rate, and better cyclability than that of the O3 61 structure. 6 62 In 1999, Paulsen and Dahn reported an exhaustive study on 63 P2-sodium manganese oxide compounds 7 after reports from

Improving the Performance of Layered Oxide Cathode Materials with Football‐Like Hierarchical Structure for Na‐Ion Batteries by Incorporating Mg2+ into Vacancies in Na‐Ion Layers

“…On one hand, XRD has been used to prove that a low current density can generate more Cmcm phase in Mg‐substituted P2‐Na 0.7 MnO 2 , which leads to a higher capacity . Higher current density causes a lower utilization of materials and generates little Cmcm phase to result in a lower capacity . Further studies demonstrate that Mg substitution in TM sites can promote the TM and Na + /vacancy disorder, faster diffusion kinetics, and can lead to improved rate performance and better capacity retention .…”

“…However, the Mg‐incorporated Mg05 cathode can exhibit much higher specific capacities of approximately 150, 126, 93, 73, and 60 mAh g −1 , respectively. On one hand, XRD has been used to prove that a low current density can generate more Cmcm phase in Mg‐substituted P2‐Na 0.7 MnO 2 , which leads to a higher capacity . Higher current density causes a lower utilization of materials and generates little Cmcm phase to result in a lower capacity .…”

Layered Oxide Cathodes for Sodium‐Ion Batteries: Phase Transition, Air Stability, and Performance

“…[98] On the other hand, Mg-substituted P2-Na 2/3 Mn 1−x Mg x O 2 (0 ≤ x ≤ 0.2) demonstrated smoother electrochemistry, improved capacity retention, and better rate performance with a fewer distinct electrochemical processes. [44,99,100] The presence of Mg on the transition metal lattice leads to fewer JahnTeller distorted Mn 3+ ions as well as disrupts potential Mn 3+ / Mn 4+ ordering. In addition, the greater number of Na + present at the end of charge, when Mg is substituted in the compound, increases the voltage range over which the P2 phase is stable and delays the occurrence of oxygen layer glides leading to the formation of an OP4 phase.…”

Recent Progress of Layered Transition Metal Oxide Cathodes for Sodium‐Ion Batteries

“…The effects of Mg doping on the structural stability and electrochemical performance of P2‐Na 2/3 MnO 2 were systematically investigated through comparison among three compositions: Na 2/3 Mn 1‐ y Mg y O 2 ( y = 0.0, 0.05, and 0.1), as shown in Figure . The Mg addition reduced the amount of Mn 3+ Jahn–Teller centers and delayed the occurrence of the phase transition at high voltage, leading to a stabilized P2 phase with fewer distinct electrochemical variations, resulting in higher capacity retention and improved rate performance . The sample with 5% Mg retained a reversible capacity of 140 mAh g −1 after 50 cycles at 1 A g −1 , and 106 mAh g −1 at 5 A g −1 .…”