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Research Interests

New type of cathode materials based on manganese
1. Spinel LiMn2O4
Spinel cathode LiMn2O4 has drawn much attention due to low cost, low toxicity, abundant resource and acceptable safety compared to the commercial LiCoO2. It becomes one of promising cathode materials with high performance for lithium rechargeable batteries. However, remarkable capacity fading during cycling, especially at elevated temperatures, prevents its commercial application. In general, the capacity loss is ascribed to Jahn-Teller distortion, dissolution of Mn ions and electrolyte oxidation. The design of microparticles with spherical-like morphology, of which the (111) plane linked the adjacent equivalent planes with curved face, is found effectively to address the issue. 

 
2. Olivine LiMnPO4
Lithium manganese phosphate with an ordered olivine structure, LiMnPO4, has attracted much attention as promising new cathode material of lithium-ion batteries because the Mn3+/Mn2+ redox couple in the olivine framework is positioned at 4.1 V versus Li/Li+, which is compatible to commercial 4 V class cathodes such as layered LiCoO2 and spinel LiMn2O4. Furthermore, the theoretical energy density is 1.2 times larger than that of LiFePO4, and the use of well-known electrolytes such as propylene carbonate, ethylene carbonate, and dimethoxyethane is suitable for LiMnPO4. However, the electrochemical performance of LiMnPO4 is poor due to the slow lithium diffusion kinetics within the grains and the low intrinsic electronic conductivity. These drawbacks may be overcame by the use of very fine particles of LiMnPO4, which results in a shortening of both the conduction path of electrons and the diffusion path of Li-ions. The obtained nano-sized LiMnPO4 shows a high discharge capacity value of nearly 160 mAh g-1 at 0.5C. 

 
3. Layered Li[Li1/3Mn2/3]O2-LiMO2 (M = Mn, Ni, and/or Co)
Layered lithium- and manganese-enriched oxides, which are the solid solutions between layered Li[Li1/3Mn2/3]O2 (also designated as Li2MnO3) and LiMO2 (M=Ni, Co, and Mn), have received much attention as some of them exhibit much higher capacity with lower cost and better safety than the currently used LiCoO2 cathode. For example, Li1.2Ni0.2Mn0.6O2 and Li1.2Ni0.13Co0.13Mn0.54O2 can deliver reversible discharge capacities as high as 250 mAh g-1 when cycled between 2.0 and 4.8 V (vs. Li+/Li). Via an optimization in composition of Li-rich oxides, a high discharge capacity of 300 mAh g-1 can be realized at room temperature under a current density of 25 mA g-1. The bid to decrease the initial irreversible capacity loss is on going. 

Graphene
Scalable production of graphene
Using an innovative preparation method, graphene can be produced in a large scale and with low cost. The mass-produced graphene has a mean thickness of 3 nm and a high conductivity of ~ 1000 S/cm, which has great potential in the application in energy storage, functional composite materials, chemical engineering, etc. Recently, a pilot line with the capacity of 30 tons of graphene per year has been established, which enables us to provide high quality graphene to clients all over the world. 

 
Graphene conductive additive
Reducing internal resistivity is essential to improve high rate capability, power density, cycling life and safety of high power Li-ion batteries (LIBs). The conductive additives play crucial roles as they help to raise electric conductivity of electrodes. Graphene, which has excellent conductivity and unique 2D nanostructure, is able to construct a 3D conductive network that has perfect contact with the active materials. Therefore, graphene is highly anticipated to be a superior conductive additive for advanced batteries. Based on the new technology to produce graphene in a large quantity, we have developed commercial graphene conductive additives with excellent performances. The new graphene conductive additives are greatly suitable for high power Li-ion batteries. 
Graphene based electrode materials for Li-ion batteries
The unique features of graphene, including its ultra-high conductivity, 2D nanostructure and its structural flexibility, impart this novel carbon material with great potential to enhance the electrochemical performance of electrode materials for Li-ion batteries. Several cathode and anode materials, such as LiFePO4, SnO2 and silicon have been successfully modified by graphene in our group. The obtained composite electrode materials exhibits remarkably improved high rate capability, specific capacity and cycling life. 

 
 
All-solid-state LIBs
1. Research on Solid Electrolytes with High Li-ion Conductivity 

In nanostructured materials with ionic conductivity, the increase in mobile ionic defects in space-charge regions can account for substantial increase in conductivity. In order to examine the electrical performance of LATP nanocrystalline solid electrolyte, complex impedance plots are measured, and the total resistances (Rb+Rgb) and the bulk resistances (Rb) of the samples are obtained from the right and left intercepts of the semicircle with the real axis in the plots, respectively. It is noteworthy that the bulk resistance remains constant for the three samples, while their grain boundary resistances change obviously. The maximum room temperature conductivity, 3.25×10-3 S/cm and 1.12×10-3 S/cm of σbulk and σtotal, respectively are obtained for the fully dense specimen (as listed in Table), which are the highest values for Li+-ion inorganic conductors as reported. And the minimum of activation energies (Ea), 0.25 eV, is obtained for SPS650+A sample.
 
 
Oxide-based electrolytes such as LIPON (Li2.9PO3.3N0.46) and Li1.4Al0.4Ti1.6(PO4)3 have to be utilized as thin films to compensate for their very low total ionic conductivities resulted from grain boundary effect, and their limited cell capacity was one of the disadvantages of all-solid-state or thin-film batteries. Therefore, the development of highly conductive solid electrolytes is imperative to create large-sized, all-solid-state batteries with high capacity. It is obviously seen in Figure that the total conductivity was almost the same as the bulk conductivity in the sulfide electrolytes, which was called thio-LISICON. The conductivity for the thio-LISICON was hardly influenced by the grain boundary layer. On the other hand, sulfide glasses and glass-ceramics in the systems of Li2S-SiS2 and Li2S-P2S5 are known to be lithium-ion conductors with high conductivities over 10-4 S cm-1 at room temperature.
The conductivities of the thio-LISICON are in the order of 10-3 S cm-1 at room temperature with very low ion conducting activation energies (Eα) of 0.22~0.27 eV. The Eα value was even lower than the minimum of Eα for all oxide solid electrolytes. The glass-ceramics materials prepared by crystallization of the Li2S-P2S5 glasses show extremely high ambient temperature conductivities of 3.2´10-3 S cm-1 as compressed powders, which is the highest ionic conductivity at ambient temperature of all lithium-ion conductors reported so far. At the same time, the system of Li2S-GeS2-P2S5 ceramics was highly stable against electrochemical oxidation with a wide electrochemical window of 10 V. The high conductivity and chemical stability towards the lithium metal anode identifies the thio-LISICON system as the most suitable candidate electrolyte for all-solid-state lithium rechargeable batteries. 
2. Study on All-solid-state Lithium Batteries with High Power Density
 
Lithium-ion batteries have been suffering from the safety issue originated from the combustible non-aqueous electrolytes. Nonflammable inorganic solid electrolytes will make them completely free from the issue. This advantage is becoming more and more important, because much bigger batteries are now required to be mounted in EVs and HEVs, where the safety issue is more serious.
In all-solid-state lithium cells, rate-determining factor was considered to be a highly-developed space-charge layer on the electrolyte side at the interface, which is generated by difference in electrochemical potential of lithium ions between the LiCoO2 and developed by electronic conduction in the LiCoO2. Because lithium ions are depleted there, it should be high in the resistance to be rate-limiting. In order to suppress the development, we interposed a thin layer of an oxide solid electrolyte at the interface as buffer layers, which drastically increased the power density. The effects of Al-substitution in LiCoO2 on the electrode properties were investigated in a sulfide solid electrolyte as shown in above Figures. The substitution improved the rate capability to enable the LiAlxCo1-xO2 to deliver a discharge capacity 84 mAh•g−1 at 5 C discharge. The improvement was considered to come from formation of Al-rich layer on the LiAlxCo1-xO2 particles, which acts as a buffer layer to reduce the interfacial resistance. 

Soft Computing Techniques for Multiple Complemented Energy Storage Control