Controllable Charge Capacity Using a Black Additive for High-Energy-Density Sodium-Ion Batteries
Authors: Jo Chang Heum , Lee Kisoo , Myung Seung-Taek 
Published: February 2020

Introduction

Sodium-deficient P2 or P′2 type layered materials are known to deliver high capacity with acceptable capacity retention. However, the initial charge capacity is substantially lower than the discharge capacity because of the insufficient amount of sodium in their crystal structure, hindering practical application of these materials as cathodes in sodium-ion batteries (SIBs). This limitation can be overcome by introducing a sacrificial salt additive, which participates in the electrochemical oxidation reaction by releasing enough sodium ions to compensate for the insufficient sodium content in the cathode material. Herein, the sacrificial salt NaNO2 was blended with a high-capacity orthorhombic P′2 type Na2/3[Co0.05Mn0.95]O2 cathode material, increasing the initial charge capacity from 154 to 210 mA h g-1. During electrochemical oxidation, the NaNO2 was oxidatively decomposed by the following reaction: NaNO2 → NO2 + Na+ + e-, where NO2 is an oxidizer that enables full desodiation to Na0[Co0.05Mn0.95]O2. The first coulombic efficiency of Na2/3[Co0.05Mn0.95]O2 was improved from 1.38 to 0.98 by virtue of the sacrificing and oxidizing roles of NaNO2.

Experimental

P’2-type layered Na2/3[Co0.05Mn0.95]O2 was synthesized via spray pyrolysis. Stoichiometric amounts of manganese nitrate tetra-hydrate, cobalt nitrate hexahydrate, and sodium nitrate were dissolved in distilled water at 25 °C. Citric acid as a chelating agent and sucrose as a particle agglomeration inhibitor were added to the prepared aqueous solution at a molar ratio of the starting material : citric acid : sucrose of 1 : 0.2 : 0.05. The concentration of the starting solution was 0.08 M. The resulting aerosol stream was introduced into a vertical quartz reactor heated to 400 °C. The as-received precursor powders were calcined at 1200 °C for 10 h in a furnace in a dry air atmosphere with a flow rate of 300 mL min-1 and subsequently cooled to 25 °C. NaNO2 was intimately blended with the conducting agents by ball-milling at a weight ratio of 1 : 1, yielding a black-colored powder. The as-synthesized Na2/3[Co0.05Mn0.95]O2 and the mixture of NaNO2 and conducting agents were homogeneously blended in an agate mortar.

Results and Discussion

The introduction of a sacrificing agent, NaNO2, into sodium-deficient P2- and P’2-type layered cathode materials helped to overcome their intrinsic drawback of low charge capacities. The additive caused the release of additional sodium ions during electrochemical oxidation, which, in turn, provided additional capacity upon charging, bringing the first charge capacity closer to the discharge capacity. As a result, a high energy density of approximately 300 Wh kg-1 (calculated based on the cathode material) was achieved for the NaNO2/Na2/3[Co0.05Mn0.95]O2//hard carbon full cell. Supplementary experimental studies revealed that additional sodium ions were intercalated into the hard carbon. Hence, the high-capacity P’2 layered cathode can be used as a cathode material for high-energy-density SIBs. Details will be mentioned at the conference site.

Figure 1