Tuning the Phase Stability of Sodium Metal Pyrophosphates for Synthesis of High Voltage Cathode Materials

Properties of the electrode materials are strongly influenced by their crystal structures, yet there is still a lack of design principles to control the polymorphism, showing multiple structures for a given composition with varying battery performance. Here, the underlying mechanism that governs the phase stability of Na₂CoP₂O₇, which has two polymorphs with different electrochemical properties, and a strategy to control it via transition metal substitution are investigated. It is found that the relative stability between the triclinic and orthorhombic polymorphs of Na₂MP₂O₇ (M = transition metals) is determined by two factors, the ionic size and crystal field stabilization energy. On the basis of this understanding, a computational strategy is devised for selecting the optimal substituents to produce a desired polymorph, from which the introduction of Ca, Ni, or Mn into Na₂CoP₂O₇ is identified to stabilize the preferred triclinic phase that has a higher voltage than the orthorhombic counterpart. This prediction of selective synthesis of a particular polymorph for improved battery performance is successfully verified by experimental syntheses, characterization, and electrochemical measurements. We expect that the current strategy can be generalized for other materials synthesis in which the functionalities of materials are sensitively dependent on the crystal polymorphs

Anomalous Manganese Activation of a Pyrophosphate Cathode in Sodium Ion Batteries: A Combined Experimental and Theoretical Study

Sodium ion batteries (SIBs) have many advantages such as the low price and abundance of sodium raw materials that are suitable for large-scale energy storage applications. Herein, we report an Mn-based pyrophosphate, Na₂MnP₂O₇, as a new SIB cathode material. Unlike most Mn-based cathode materials, which suffer severely from sluggish kinetics, Na₂MnP₂O₇ exhibits good electrochemical activity at ∼3.8 V vs Na/Na⁺ with a reversible capacity of 90 mAh g^–1 at room temperature. It also shows an excellent cycling and rate performance: 96% capacity retention after 30 cycles and 70% capacity retention at a c-rate increase from 0.05C to 1C. These electrochemical activities of the Mn-containing cathode material even at room temperature with relatively large particle sizes are remarkable considering an almost complete inactivity of the Li counterpart, Li₂MnP₂O₇. Using first-principles calculations, we find that the significantly enhanced kinetics of Na₂MnP₂O₇ is mainly due to the locally flexible accommodation of Jahn–Teller distortions aided by the corner-sharing crystal structure in triclinic Na₂MnP₂O₇. By contrast, in monoclinic Li₂MnP₂O₇, the edge-sharing geometry causes multiple bonds to be broken and formed during charging reaction with a large degree of atomic rearrangements. We expect that the similar computational strategy to analyze the atomic rearrangements can be used to predict the kinetics behavior when exploring new cathode candidates

The High Performance of Crystal Water Containing Manganese Birnessite Cathodes for Magnesium Batteries

Rechargeable magnesium batteries have lately received great attention for large-scale energy storage systems due to their high volumetric capacities, low materials cost, and safe characteristic. However, the bivalency of Mg²⁺ ions has made it challenging to find cathode materials operating at high voltages with decent (de)­intercalation kinetics. In an effort to overcome this challenge, we adopt an unconventional approach of engaging crystal water in the layered structure of <i>Birnessite</i> MnO₂ because the crystal water can effectively screen electrostatic interactions between Mg²⁺ ions and the host anions. The crucial role of the crystal water was revealed by directly visualizing its presence and dynamic rearrangement using scanning transmission electron microscopy (STEM). Moreover, the importance of lowering desolvation energy penalty at the cathode–electrolyte interface was elucidated by working with water containing nonaqueous electrolytes. In aqueous electrolytes, the decreased interfacial energy penalty by hydration of Mg²⁺ allows <i>Birnessite</i> MnO₂ to achieve a large reversible capacity (231.1 mAh g^–1) at high operating voltage (2.8 V vs Mg/Mg²⁺) with excellent cycle life (62.5% retention after 10000 cycles), unveiling the importance of effective charge shielding in the host and facile Mg²⁺ ions transfer through the cathode’s interface