Matthew J. McDonald

P2-type Na0.66Ni0.33–xZnxMn0.67O2 as new high-voltage cathode materials for sodium-ion batteries

P2-Na0.67Ni0.33−x Cu x Mn0.67O2 (x = 0, 0.02, 0.04, 0.06, 0.08) cathode materials have been synthesized via acetate decomposition method. The elementary composition and crystal structure of the powders are studied in detail using inductively coupled plasma-atomic emission spectrometry (ICP-AES) and X-ray diffraction (XRD). XRD results demonstrate that Cu2+ ions have been incorporated into the crystal structure successfully and the P2-type structure remains unchanged after substitution. According to XPS data, Cu substitution does not change the valence states of Ni and Mn, whose predominant oxidation states in Na-Ni-Mn-O structure remains +2 and +4. The introduction of Cu2+ can effectively suppress P2-O2 phase transformation when charging to 4.5 V, and significantly improve rate performance and cyclic stability compared to the undoped material. The P2-Na0.67Ni0.27Cu0.06Mn0.67O2 sample can deliver an initial discharge capacity of 211.6 mAh g−1 at 10 mAh g−1 between 1.5 and 4.5 V, and a capacity retention of 93.9% after 10 cycles. Moreover, it can also deliver a discharge capacity of 115.2 mAh g−1 at 100 mAh g−1. In addition, electrochemical impedance spectroscopy (EIS) reveals that P2-Na0.67Ni0.27Cu0.06Mn0.67O2 cathode exhibits a higher electronic conductivity and faster sodium ion diffusion velocity than that of undoped sample. These results show that P2-Na0.67Ni0.27Cu0.06Mn0.67O2 is a promising high-voltage cathode material for sodium-ion batteries.

The synergistic effects of Al and Te on the structure and Li+-mobility of garnet-type solid electrolytes

The cubic garnet-type solid electrolyte Li7La3Zr2O12 with aliovalent doping exhibits a high ionic conductivity. However, the synergistic effects of aliovalent co-doping on the ionic conductivity of garnet-type electrolytes have rarely been examined. In this work, the synergistic effects of co-dopants Al and Te on the ionic conductivity of garnets were investigated using X-ray diffraction (XRD), 27Al/6Li Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR), Energy Dispersive X-ray Spectroscopy (EDS), Neutron Powder Diffraction (NPD) and Alternating Current (AC) impedance measurements. It was shown that co-dopants Al and Te stabilized the cubic lattice of Li7−2x−3yAlyLa3Zr2−xTexO12 with specific Al/Te ratios, where additional Al had to be included in the structure if the amount of doped Te content x was below 0.5. In the Al and Te co-doped crystal structure, Al was incorporated into the tetrahedral 24d sites of lithium and Te occupied 16a sites of Zr. It was revealed that the occupancy of the latter could suppress the insertion of Al. High-resolution 6Li MAS NMR was able to differentiate the two lithium sites of interest in the garnet structure. Furthermore, it was shown that the mobility of Li ions at 24d sites mainly determined the bulk conductivities of garnet-type electrolytes.

Zero-Strain Na2FeSiO4 as Novel Cathode Material for Sodium-Ion Batteries

A new cubic polymorph of sodium iron silicate, Na2FeSiO4, is reported for the first time as a cathode material for Na-ion batteries. It adopts an unprecedented cubic rigid tetrahedral open framework structure, i.e., F4̅3m, leading to a polyanion cathode material without apparent cell volume change during the charge/discharge processes. This cathode shows a reversible capacity of 106 mAh g(-1) and a capacity retention of 96% at 5 mA g(-1) after 20 cycles.

Insights into the Effects of Zinc Doping on Structural Phase Transition of P2-Type Sodium Nickel Manganese Oxide Cathodes for High-Energy Sodium Ion Batteries

P2-type sodium nickel manganese oxide-based cathode materials with higher energy densities are prime candidates for applications in rechargeable sodium ion batteries. A systematic study combining in situ high energy X-ray diffraction (HEXRD), ex situ X-ray absorption fine spectroscopy (XAFS), transmission electron microscopy (TEM), and solid-state nuclear magnetic resonance (SS-NMR) techniques was carried out to gain a deep insight into the structural evolution of P2-Na0.66Ni0.33-xZnxMn0.67O2 (x = 0, 0.07) during cycling. In situ HEXRD and ex situ TEM measurements indicate that an irreversible phase transition occurs upon sodium insertion-extraction of Na0.66Ni0.33Mn0.67O2. Zinc doping of this system results in a high structural reversibility. XAFS measurements indicate that both materials are almost completely dependent on the Ni(4+)/Ni(3+)/Ni(2+) redox couple to provide charge/discharge capacity. SS-NMR measurements indicate that both reversible and irreversible migration of transition metal ions into the sodium layer occurs in the material at the fully charged state. The irreversible migration of transition metal ions triggers a structural distortion, leading to the observed capacity and voltage fading. Our results allow a new understanding of the importance of improving the stability of transition metal layers.

Enhancing the energy density of safer Li-ion batteries by combining high-voltage lithium cobalt fluorophosphate cathodes and nanostructured titania anodes

Recently, Li-ion batteries have been heavily scrutinized because of the apparent incompatibility between safety and high energy density. This work report a high voltage full battery made with TiO2/Li3PO4/Li2CoPO4F. The Li2CoPO4F cathode and TiO2 anode materials are synthesized by a sol–gel and anodization methods, respectively. X-ray diffraction (XRD) analysis confirmed that Li2CoPO4F is well-crystallized in orthorhombic crystal structure with Pnma space group. The Li3PO4-coated anode was successfully deposited as shown by the (011) lattice fringes of anatase TiO2 and (200) of γ-Li3PO4, as detected by HRTEM. The charge profile of Li2CoPO4F versus lithium shows a plateau at 5.0 V, revealing its importance as potentially high-voltage cathode and could perfectly fit with the plateau of anatase anode (1.8–1.9 V). The full cell made with TiO2/Li3PO4/Li2CoPO4F delivered an initial reversible capacity of 150 mA h g−1 at C rate with good cyclic performance at an average potential of 3.1–3.2 V. Thus, the full cell provides an energy density of 472 W h kg−1. This full battery behaves better than TiO2/Li2CoPO4F. The introduction of Li3PO4 as buffer layer is expected to help the cyclability of the electrodes as it allows a rapid Li-ion transport.

Exploring the working mechanism of Li+ in O3-type NaLi0.1Ni0.35Mn0.55O2 cathode materials for rechargeable Na-ion batteries

Na-ion batteries (NIBs) have recently attracted much attention, due to their low cost and the abundance of sodium resources. In this work, NaLi0.1Ni0.35Mn0.55O2 as a promising new kind of cathode material for Na-ion batteries was synthesized by a co-precipitation method. Powder XRD patterns show that the sample has a primary O3-type structure after Li+ substitution. The material delivers excellent electrochemical performance, with an initial discharge specific capacity of 128 mA h g−1 and a capacity retention of 85% after 100 cycles at a rate of 12 mA g−1 in the voltage range of 2.0–4.2 V. In a widened voltage range of 1.5–4.3 V, the specific capacity can reach up to 160 mA h g−1. The structural stability of the material is substantially improved compared with lithium-free NaNi0.5Mn0.5O2, which can be attributed to the formation of an O′3 phase caused by Li-substitution, as proven by in situ XRD and solid state NMR (ss-NMR) measurements.

Polyanion Compounds as Cathode Materials for Li-Ion Batteries

The development of high energy density Li-ion batteries depends on finding electrode materials that can meet increasingly stringent demands, in particular cathode materials (Goodenough and Kim in Chem Mater 22:587–603, 2010; Tarascon and Armand in Nature 414:359–367, 2001). Cathodes are not only the primary factor producing the working potential of Li-ion batteries, but also determine the number of Li ions (i.e., the practical capacity) which can be utilized. Due to LiFePO4 having succeeded as a prime example of high powered Li-ion battery material, polyanion-type compounds have attracted wide interests in the field of cathode research for the last two decades.