Unveiling the Role of Co in Improving the High-Rate Capability and Cycling Performance of Layered Na_0.7Mn_0.7Ni_0.3–xCo_xO₂ Cathode Materials for Sodium-Ion Batteries

Abstract

Co substitution has been extensively used to improve the electrochemical performances of cathode materials for sodium-ion batteries (SIBs), but the role of Co has not been well understood. Herein, we have comprehensively investigated the effects of Co substitution for Ni on the structure and electrochemical performances of Na0.7Mn0.7Ni0.3-xCoxO2 (x = 0, 0.1, 0.3) as cathode materials for SIBs. In comparison with the Co-free sample, the high-rate capability and cycle performance have been greatly improved by the substitution of Co, and some new insights into the role of Co have been proposed for the first time. With the substitution of Co(3+) for Ni(2+) the lattice parameter a decreases; however, c increases, and the d-spacing of the sodium-ion diffusion layer has been enlarged, which enhances the diffusion coefficient of the sodium ion and the high-rate capability of cathode materials. In addition, Co substitution shortens the bond lengths of TM-O (TM = transition metal) and O-O due to the smaller size of Co(3+) than Ni(2+), which accounts for the decreased thickness and volume of the TMO6 octahedron. The contraction of TM-O and O-O bond lengths and the shrinkage of the TMO6 octahedron improve the structure stability and the cycle performance. Last but not least, the aliovalent substitution of Co(3+) for Ni(2+) can improve the electronic conductivity during the electrochemical reaction, which is also favorable to enhance the high-rate performance. This study not only unveils the role of Co in improving the high-rate capability and the cycle stability of layered Na0.7Mn0.7Ni0.3-xCoxO2 cathode materials but also offers some new insights into designing high performance cathode materials for SIBs.

Utilizing Co2+/Co3+ Redox Couple in P2‐Layered Na0.66Co0.22Mn0.44Ti0.34O2 Cathode for Sodium‐Ion Batteries

“…By comparing with the spectra of the standard transition metal oxide compounds, it is revealed that the average valence states of the Co, Mn, and Ti ions in the as‐synthesized NCMT‐2 are 2+, 3.7+, and 4+. It is very surprising that Co ions can be stabilized at 2+ in the P2‐type layer‐structured compounds, since the valence state of Co ions in most layer‐structured cathode materials is 3+ 12, 23, 39. In addition, the valence state of Mn ions in NCMT‐2 is 3.7+, which is higher than Mn 3+ in tunnel‐structured Na 0.66 Mn 0.66 Ti 0.34 O 2 18.…”

“…In addition, the valence state of Mn ions in NCMT‐2 is 3.7+, which is higher than Mn 3+ in tunnel‐structured Na 0.66 Mn 0.66 Ti 0.34 O 2 18. It is well known that high‐spin Mn 3+ with the electron configuration of (t 2g ) 3 (e g ) 1 in the 3D orbital exhibits very strong Jahn–Teller distortion due to the single occupancy of a degenerate e g orbital 12, 22. The low valence Co 2+ substitution in NCMT‐2 introduces more Mn 4+ because the required charge balance, which resulted the average Mn 3.7+ in the P2‐type structure.…”

“…Over the past years, various cathode materials have been investigated, including layered oxides,9, 10, 11, 12, 13, 14, 15 tunnel‐structured oxides,16, 17, 18 polyanion compounds,19, 20 and organic compounds 21. Among them, layered oxides are promising cathode materials with a general formula of Na x TMO 2 (TM = transition metals, such as Co, Ni, Mn, Cr, Fe, and Cu),9, 12, 22, 23, 24 which can be categorized into two main groups: P2‐type and O3‐type. P2 and O3 are defined according to the oxygen stacking sequence and coordination environment of the alkali ions 25, 26.…”

“…It was found that P2‐type Na 2/3 Co 2/3 Mn 1/3 O 2 displays a reversible 0.5 Na + intercalation between 1.5 and 4.0 V. The good reversibility is attributed to the fact that the introduced low‐spin Co 3+ ions can stabilize Mn ions at a valence state of 4+ 23, 34. Li et al reported that Ni and Co substitution in Na 0.7 Mn 0.7 Ni 0.3− x Co x O 2 could enhance sodium diffusion coefficient and high‐rate capability by shortening the bond lengths of TM—O and O—O 12. In addition, introducing Ti ions into transition metal layer can reduce the structural distortions during charge and discharge processes and suppress the formation of a secondary rock salt phase at high voltage charge 10, 22…”

“…In a large number of P2‐type layered oxides,12, 22, 23, 24, 36, 37, 38 transition metal ordering is mainly determined by the difference between the ionic radii of M1 and M2, in which small difference (usually less than 15%) favors to form disordered arrangement if M1 and M2 content is close to a rational rate, while a larger difference (greater than 15%) favors an ordered arrangement. In the case of charge ordering, it is determined by the redox potential of M1 and M2.…”