Huiming Wu

Formation of the spinel phase in the layered composite cathode used in Li-Ion batteries

Pristine Li-rich layered cathodes, such as Li(1.2)Ni(0.2)Mn(0.6)O(2) and Li(1.2)Ni(0.1)Mn(0.525)Co(0.175)O(2), were identified to exist in two different structures: LiMO(2)R3[overline]m and Li(2)MO(3)C2/m phases. Upon 300 cycles of charge/discharge, both phases gradually transform to the spinel structure. The transition from LiMO(2)R3[overline]m to spinel is accomplished through the migration of transition metal ions to the Li site without breaking down the lattice, leading to the formation of mosaic structured spinel grains within the parent particle. In contrast, transition from Li(2)MO(3)C2/m to spinel involves removal of Li(+) and O(2-), which produces large lattice strain and leads to the breakdown of the parent lattice. The newly formed spinel grains show random orientation within the same particle. Cracks and pores were also noticed within some layered nanoparticles after cycling, which is believed to be the consequence of the lattice breakdown and vacancy condensation upon removal of lithium ions. The AlF(3)-coating can partially relieve the spinel formation in the layered structure during cycling, resulting in a slower capacity decay. However, the AlF(3)-coating on the layered structure cannot ultimately stop the spinel formation. The observation of structure transition characteristics discussed in this paper provides direct explanation for the observed gradual capacity loss and poor rate performance of the layered composite. It also provides clues about how to improve the materials structure in order to improve electrochemical performance.

Free-Standing Hierarchically Sandwich-Type Tungsten Disulfide Nanotubes/Graphene Anode for Lithium-Ion Batteries

Transition metal dichalcogenides (TMD), analogue of graphene, could form various dimensionalities. Similar to carbon, one-dimensional (1D) nanotube of TMD materials has wide application in hydrogen storage, Li-ion batteries, and supercapacitors due to their unique structure and properties. Here we demonstrate the feasibility of tungsten disulfide nanotubes (WS2-NTs)/graphene (GS) sandwich-type architecture as anode for lithium-ion batteries for the first time. The graphene-based hierarchical architecture plays vital roles in achieving fast electron/ion transfer, thus leading to good electrochemical performance. When evaluated as anode, WS2-NTs/GS hybrid could maintain a capacity of 318.6 mA/g over 500 cycles at a current density of 1A/g. Besides, the hybrid anode does not require any additional polymetric binder, conductive additives, or a separate metal current-collector. The relatively high density of this hybrid is beneficial for high capacity per unit volume. Those characteristics make it a potential anode material for light and high-performance lithium-ion batteries.

Fluorinated electrolytes for 5 V lithium-ion battery chemistry

An electrolyte based on fluorinated carbonate solvents was evaluated with high voltage cathode materials at elevated temperature. The theoretically high anodic stability of these new electrolytes was supported by electrochemical evaluation results using LiNi0.5Mn1.5O4/Li and LiNi0.5Mn1.5O4/Li4Ti5O12 electrochemical couples. Fluorinated carbonate appears to be a suitable electrolyte candidate for transition metal oxide cathodes at high voltage (5 V vs. Li+/Li).

Effect of the size-selective silver clusters on lithium peroxide morphology in lithium–oxygen batteries

Lithium-oxygen batteries have the potential needed for long-range electric vehicles, but the charge and discharge chemistries are complex and not well understood. The active sites on cathode surfaces and their role in electrochemical reactions in aprotic lithium-oxygen cells are difficult to ascertain because the exact nature of the sites is unknown. Here we report the deposition of subnanometre silver clusters of exact size and number of atoms on passivated carbon to study the discharge process in lithium-oxygen cells. The results reveal dramatically different morphologies of the electrochemically grown lithium peroxide dependent on the size of the clusters. This dependence is found to be due to the influence of the cluster size on the formation mechanism, which also affects the charge process. The results of this study suggest that precise control of subnanometre surface structure on cathodes can be used as a means to improve the performance of lithium-oxygen cells.

Effectively suppressing dissolution of manganese from spinel lithium manganate via a nanoscale surface-doping approach

The capacity fade of lithium manganate-based cells is associated with the dissolution of Mn from cathode/electrolyte interface due to the disproportionation reaction of Mn(III), and the subsequent deposition of Mn(II) on the anode. Suppressing the dissolution of Mn from the cathode is critical to reducing capacity fade of LiMn2O4-based cells. Here we report a nanoscale surface-doping approach that minimizes Mn dissolution from lithium manganate. This approach exploits advantages of both bulk doping and surface-coating methods by stabilizing surface crystal structure of lithium manganate through cationic doping while maintaining bulk lithium manganate structure, and protecting bulk lithium manganate from electrolyte corrosion while maintaining ion and charge transport channels on the surface through the electrochemically active doping layer. Consequently, the surface-doped lithium manganate demonstrates enhanced electrochemical performance. This study provides encouraging evidence that surface doping could be a promising alternative to improve the cycling performance of lithium-ion batteries.

Nanostructured Lithium Nickel Manganese Oxides for Lithium-Ion Batteries

Nanostructured lithium nickel manganese oxides were investigated as advanced positive electrode materials for lithium-ion batteries designated to power plug-in hybrid electric vehicles and all-electric vehicles. The investigation included material characterization and electrochemical testing. In cell tests, the Li{sub 1.375}Ni{sub 0.25}Mn{sub 0.75}O{sub 2.4375} composition achieved high capacity (210 mAh g{sup -1}) at an elevated rate (230 mA g{sup -1}), which makes this material a promising candidate for high energy density Li-ion batteries, as does its being cobalt-free and uncoated. The material has spherical morphology with nanoprimary particles embedded in micrometer-sized secondary particles, possesses a multiphase character (spinel and layered), and exhibits a high packing density (over 2 g cm{sup -3}) that is essential for the design of high energy density positive electrodes. When combined with the Li{sub 4}Ti{sub 5}O{sub 12} stable anode, the cell showed a capacity of 225 mAh g{sup -1} at the C/3 rate (73 mA g{sup -1}) with no capacity fading for 200 cycles. Other chemical compositions, Li{sub (1+x)}Ni{sub 0.25}Mn{sub 0.75}O{sub (2.25+x/2)} (0.32 {le} x {le} 0.65), were also studied, and the relationships among their structural, morphological, and electrochemical properties are reported.

A novel concentration-gradient Li[Ni0.83Co0.07Mn0.10]O2 cathode material for high-energy lithium-ion batteries

A novel concentration-gradient Li[Ni0.83Co0.07Mn0.10]O2 cathode material was successfully synthesized viaco-precipitation, in which the core Li[Ni0.9Co0.05Mn0.05]O2 was encapsulated completely with a stable concentration-gradient layer having reduced Ni content. The electrochemical and thermal properties of the concentration-gradient Li[Ni0.83Co0.07Mn0.10]O2 were studied and compared to those of the core Li[Ni0.9Co0.05Mn0.05]O2 material alone. The concentration-gradient material had a superior lithium intercalation stability and thermal stability compared to the core material. The high capacity was delivered from the Ni-rich core Li[Ni0.9Co0.05Mn0.05]O2, and the improved thermal stability was achieved by the Ni-depleted concentration-gradient layer with outer surface composition of Li[Ni0.68Co0.12Mn0.20]O2. The concentration-gradient materials open a new era for the development of advanced Li-ion batteries with high energy density, long cycle life, and improved safety.

An effective approach to protect lithium anode and improve cycle performance for Li-S batteries.

Lithium oxalyldifluoroborate (LiODFB) has been investigated as an organic electrolyte additive to improve the cycling performance of Li-S batteries. Cell test results demonstrate that an appropriate amount of LiODFB added into the electrolyte leads to a high Coulombic efficiency. Analyses by energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and the density functional theory showed that LiODFB promotes the formation of a LiF-rich passivation layer on the lithium metal surface, which not only blocks the polysulfide shuttle, but also stabilizes the lithium surface.

Development of LiNi0.5Mn1.5O4 / Li4Ti5O12 System with Long Cycle Life

The electrochemical performance of LiNi{sub 0.5}Mn{sub 1.5}O{sub 4} (LNMO)/Li{sub 4}Ti{sub 5}O{sub 12} (LTO) cells with different designs, positive-electrode-limited, negative-electrode-limited, and positive/negative capacity ratio of {approx} (1) was investigated at different temperatures and current densities. Half-cells based on either LNMO/Li or LTO/Li exhibited an outstanding rate capability. When both electrodes were combined in a full cell configuration, the LNMO/LTO cells showed an excellent rate capability, with 86% discharge capacity retention at the 10C rate. Of the three designs, the negative-limited full cell showed the best cycling performance when discharged at the 2C rate in tests at room temperature: 98% capacity retention after 1000 cycles. The negative-limited full cell also exhibited excellent cycling characteristics in tests at 55 C: 95% discharge capacity retention after 200 cycles. These results clearly demonstrate that the LNMO/LTO system with a negative-limited design is attractive for plug-in hybrid electric vehicles, where a long cycle life and a reasonable power are needed.

Effect of Cobalt Incorporation and Lithium Enrichment in Lithium Nickel Manganese Oxides

Candidate cathode materials of cobalt-incorporated and lithium-enriched Li{sub (1+x)}Ni{sub 0.25}Co{sub 0.15}Mn{sub 0.6}O{sub (2.175+x/2)} (x=0.225-0.65) have been prepared by a coprecipitation method and a solid-state reaction. We systematically investigated the effect of both cobalt presence and lithium concentration on the structure, physical properties, and electrochemical behavior of the studied samples. The electrochemical performance of the cobalt-containing compounds showed much less dependence on the variation in the lithium amounts compared to the cobalt-free counterpart. The study demonstrated that even with cobalt incorporation, proper lithium content is the key to desirable cathode materials with nanostructured primary particles that are indispensable to achieve high capacity and high rate capability and, therefore, both improved energy and power densities for lithium-ion batteries.