Structural, Electrochemical and Thermal Studies on Na x CrO2 for Na-Ion Batteries

Abstract

Layered Na x MeO2 (Me = transition metal) compounds have been intensively studied as electrode materials for Na-ion batteries. Among the sodium insertion materials, α-NaFeO2-type NaCrO2 (or classified as the O3-type layered phase) is a promising candidate as a positive electrode material for rechargeable sodium batteries due to its cycle performance and excellent thermal stability.1, 2 Our group reported that O3-type NaCrO2 electrode delivered reversible capacity of ca. 120 mAh g-1, corresponding to 0.48 mol of Na extraction by charge to 3.6 V but the reversibility was lost by further charge to 4.5 V.3 This fact was thought to be presumably caused by an irreversible structural change, accompanied with more than 0.5 mol of Na extraction. In this study, to understand the phase transition mechanism during charge, we examine the structural change by ex-situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Furthermore, since safety is essential for battery systems, especially for the large scale applications, the thermal stability is also studied by differential scanning calorimetry (DSC), thermogravimetry (TG), and high temperature XRD (HT-XRD). Figure 1 shows ex-situ XRD patterns of NaCrO2 composite electrodes with different sodium amount, x in Na1-x CrO2. The amount of sodium was calculated from the quantity of charge passed electrode. The XRD patterns in the range of 0.0 ≤ x ≤0.52 indicate several phase transitions similar to the observation of in-situ XRD patterns in the literature.4 As-prepared NaCrO2 (x = 0) indexed as the O3-type layered phase with space group, R-3m changes into O’3-type distorted layered structure in the range of 0.1 < x <0.3. Then the O’3-type layered phase transforms into P’3-type distorted layered structure in the range of 0.26 ≤ x ≤ 0.52, where sodium ions locate at triangular prismatic sites. After further charge to x = 0.87, the XRD pattern is indexed as O3-type layered structure with the quite short interslab distance. Interestingly, relatively low and high intensity of 003 and 101 lines, respectively, is observed compared to that of 104 line, which suggests that chromium ions partially accommodate at tetrahedral and octahedral sites in sodium layers. XAS spectra also support the existence of chromium ions at tetrahedral sites for the sample discharged to 2.5 V after charge to 4.5 V. These results reveal that irreversible migration of chromium ions from CrO2 slabs to sodium layers occurs during charge above 3.6 V and the irreversible structural change would limit the reversible range. On the other hand, DSC and TG data for Na0.5CrO2 exhibit a high decomposition temperature compared to Na0.5CoO2 and Li0.5CoO2. The HT-XRD of Na0.5CrO2 is used to study phase transition mechanisms during the heating process. The heating beyond 250 °C results in the segregation of Na rich and Na deficient P3 phases. The Na deficient P3-phase is thermally decomposed (approximately 400 ºC) by which oxygen is released from Na0.5CrO2. The high decomposition temperature indicates superior thermal stability of Na0.5CrO2. From these results, we will further discuss thermal stability and charge/discharge mechanisms of sodium insertion layered oxides as electrode materials for Na-ion batteries.