Guiming Zhong

Nanostructured Black Phosphorus/Ketjenblack–Multiwalled Carbon Nanotubes Composite as High Performance Anode Material for Sodium-Ion Batteries

Sodium-ion batteries are promising alternatives to lithium-ion batteries for large-scale applications. However, the low capacity and poor rate capability of existing anodes for sodium-ion batteries are bottlenecks for future developments. Here, we report a high performance nanostructured anode material for sodium-ion batteries that is fabricated by high energy ball milling to form black phosphorus/Ketjenblack-multiwalled carbon nanotubes (BPC) composite. With this strategy, the BPC composite with a high phosphorus content (70 wt %) could deliver a very high initial Coulombic efficiency (>90%) and high specific capacity with excellent cyclability at high rate of charge/discharge (∼1700 mAh g(-1) after 100 cycles at 1.3 A g(-1) based on the mass of P). In situ electrochemical impedance spectroscopy, synchrotron high energy X-ray diffraction, ex situ small/wide-angle X-ray scattering, high resolution transmission electronic microscopy, and nuclear magnetic resonance were further used to unravel its superior sodium storage performance. The scientific findings gained in this work are expected to serve as a guide for future design on high performance anode material for sodium-ion batteries.

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.

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.

Promoting long-term cycling performance of high-voltage Li2CoPO4F by the stabilization of electrode/electrolyte interface

High-voltage Li2CoPO4F (∼5 V vs. Li/Li+) with double-layer surface coating has been successfully prepared for the first time. The Li3PO4-coated Li2CoPO4F shows a high reversible capacity of 154 mA h g−1 (energy density up to 700 W h kg−1) at 1 C current rate, and excellent rate capability (141 mA h g−1 at 20 C). XRD and MAS NMR results show that Li2CoPO4F can be indexed as an orthorhombic structure with space group Pnma and coexists with Li3PO4. The XPS depth profiles and TEM analysis reveal that the as-prepared material has a double-layer surface coating, with a carbon outer layer and a Li3PO4 inner layer, which greatly enhances the transfer kinetics of the lithium ions and electrons in the material and stabilizes the electrode/electrolyte interface. Using LiBOB as an electrolyte additive is another way to further stabilize the electrode/electrolyte interface, and the LiBOB has a synergistic effect with the Li3PO4 coating layer. In this way, the Li2CoPO4F cathode material exhibits excellent long-term cycling stability, with 83.8% capacity retention after 150 cycles. The excellent cycling performance is attributed to the LiBOB electrolyte additive and the Li3PO4 coating layer, both of which play an important role in stabilizing the charge transfer resistance of Li2CoPO4F upon cycling.

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.

Non-Destructive Monitoring of Charge-Discharge Cycles on Lithium Ion Batteries using 7 Li Stray-Field Imaging

Magnetic resonance imaging provides a noninvasive method for in situ monitoring of electrochemical processes involved in charge/discharge cycling of batteries. Determining how the electrochemical processes become irreversible, ultimately resulting in degraded battery performance, will aid in developing new battery materials and designing better batteries. Here we introduce the use of an alternative in situ diagnostic tool to monitor the electrochemical processes. Utilizing a very large field-gradient in the fringe field of a magnet, stray-field-imaging (STRAFI) technique significantly improves the image resolution. These STRAFI images enable the real time monitoring of the electrodes at a micron level. It is demonstrated by two prototype half-cells, graphite∥Li and LiFePO4∥Li, that the high-resolution 7Li STRAFI profiles allow one to visualize in situ Li-ions transfer between the electrodes during charge/discharge cyclings as well as the formation and changes of irreversible microstructures of the Li components, and particularly reveal a non-uniform Li-ion distribution in the graphite.

Solid-state STRAFI NMR probe for material imaging of quadrupolar nuclei

Stray field imaging (STRAFI) has provided an alternative imaging method to study solid materials that are typically difficult to obtain using conventional MRI methods. For small volume samples, image resolution is a challenge since extremely strong gradients are required to examine narrow slices. Here we present a STRAFI probe for imaging materials with quadrupolar nuclei. Experiments were performed on a 19.6 T magnet which has a fringe field gradient strength of 72 T/m, nearly 50 times stronger than commercial microimagers. We demonstrate the ability to acquire (7)Li 1D profiles of liquid and solid state lithium phantoms with clearly resolved features in the micrometer scale and as a practical example a Li ion battery electrode material is also examined.

Insights into the Electrochemical Reaction Mechanism of a Novel Cathode Material CuNi2(PO4)2/C for Li-Ion Batteries

In this work, we first report the composite of CuNi2(PO4)2/C (CNP/C) can be employed as the high-capacity conversion-type cathode material for rechargeable Li-ion batteries (LIBs), delivering a reversible capacity as high as 306 mA h g-1 at a current density of 20 mA g-1. Furthermore, CNP/C also presents good rate performance and reasonable cycling stability based on a nontraditional conversion reaction mode. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) characterizations show that CNP is reduced to form Cu/Ni and Li3PO4 during the discharging process, which is reversed in the following charging process, demonstrating that a reversible conversion reaction mechanism occurs. X-ray absorption spectroscopy (XAS) discloses that Ni2+/Ni0 exhibits a better reversibility compared to Cu2+/Cu during the conversion reaction process, while Cu0 is more difficult to be reoxidized during the recharge process, leading to capacity loss as a consequence. The fundamental understanding obtained in this work provides some important clues to explore the high-capacity conversion-type cathode materials for rechargeable LIBs.