Yarong Wang

The Design of a LiFePO4/Carbon Nanocomposite With a Core–Shell Structure and Its Synthesis by an In Situ Polymerization Restriction Method

Nano-sized electrode materials for lithium-ion batteries have attracted much attention recently because their reduced dimensions enable much higher power. However, the large electrolyte/electrode interface arising from their size leads to more undesired reactions, which result in poor cycling performance. Moreover, some nano-sized cathode materials synthesized by low-temperature methods are poorly crystalline, which also reduces their electrochemical stability. The synthesis of highly crystalline nanomaterials completely coated with conductive carbon (or a carbon shell) would be an effective means of eliminating these problems. Such a synthesis is a significant challenge, however, as the highly crystalline structure and its subsequent coating with conductive carbon have to be achieved at high temperature, where larger crystallite sizes are almost inevitable. Olivine (LiFePO4) is considered to be one of the most promising cathode materials for the next generation of lithium batteries due to its low toxicity, low cost, and high safety. However, its power performance is greatly limited by slow diffusion of lithium ions across the two-phase boundary and/or low conductivity. Many efforts have been made over the past few years to improve the power performance of LiFePO4 by using low-temperature routes to obtain tailored particles or carbon painting to improve the conductivity of the solid phase. However, these previous studies have always focused on the “nano-size” or the “coating with conductive carbon” separately, rather than considering both of them together. Various low-temperature methods (synthesis temperature below 600 8C), such as lowtemperature ceramic routes or hydrothermal syntheses, have been developed to lower the particle size of LiFePO4, although none of them have been able to ensure the conductivity of the carbon coating. Furthermore, some lowtemperature routes are not able to produce the required highly crystalline olivine structure, thus reducing the electrochemical stability of LiFePO4. The high surface area arising from the nano-size of the products also greatly increases the undesirable electrode/electrolyte reactions, which leads to a poor cycling performance From a review of previous studies of nano-sized LiFePO4 (less than 100 nm), we can see that a “perfect” cycle-life (> 200 cycles) at high charge/ discharge depth (90%) is almost unheard of. Approaches based on the thermal decomposition of carbon-containing precursors have also been widely studied for the preparation of carbon-coated LiFePO4 particles. [16–23] However, these methods generally involve a high-temperature treatment, during which an increase in crystallite size is inevitable, to ensure the conductivity of the resulting carbon materials. Accordingly, those approaches based on the thermal decomposition of carbon-containing precursors can only produce LiFePO4 particles with a partial coating of carbon (Figure 1a). As shown in Figure 1a, during the intercalation process, the electrons cannot reach all the positions where Li ion intercalation takes place, thus resulting in polarization of the electrode. In view of the one-dimensional Li ion mobility in the framework, full coating with carbon, which ensures LiFePO4 particles get electrons from all directions, could further alleviate this polarization phenomenon. According to our analysis of previous studies, the ideal structure for high-performance LiFePO4 should contain nano-size particles completely coated with conductive carbon (Figure 1b). It should be noted that many previous studies involving the synthesis of nano-sized LiFePO4 employ Fe 2+ salts as precursors. 11–13,21] However, these salts are much more expensive and unstable than Fe salts, therefore the synthesis of a nano-sized LiFePO4/carbon composite with a core–shell structure from Fe salts would be of great interest. Herein we report an in situ polymerization restriction method for the synthesis of a LiFePO4/carbon composite formed from a highly crystalline LiFePO4 core with a size of about 20–40 nm and a semi-graphitic carbon shell with a thickness of about 1–2 nm. As shown in Figure 1c, our strategy includes one in situ polymerization reaction and two typical restriction processes. The first of these restriction processes involves the addition of Fe ions to a solution containing PO4 3 ions and aniline, where it acts as both a precipitator for PO4 3 and oxidant for aniline. The reaction during this process can be summarized as Equations (1) and (2).

Synthesis and electrochemical performance of nano-sized Li4Ti5O12 with double surface modification of Ti(III) and carbon

Spinel Li4Ti5O12 nano-particles with double conductive surface modification of Ti(III) and carbon were synthesized by a facile solid-state reaction, in which the polyaniline (PANI) coated TiO2 particles and a lithium salt were used as precursors. On heat treatment under an argon atmosphere containing 5% H2, the carbonization of PANI effectively restricted the particle-size growth of Li4Ti5O12 and reduced the surface Ti(IV) into Ti(III). The surface modification combined with tailored particle size can improve the surface conductivity and shorten the Li-ion diffusion path. Furthermore, both the Ti(III) surface modification and the tailored particles (50–70nm) have the potential to increase the solid solution (single-phase insertion/extraction) during the electrochemical process. Electrochemical analysis indicated that the presence of the solid solution is beneficial for Li-ion mobility. Thereby, the prepared Li4Ti5O12 displays high power performance.

Electrochemical kinetics of the 0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2 ‘composite’ layered cathode material for lithium-ion batteries

The ‘composite’ layered material of 0.5Li2MnO3·0.5LiMn0.42Ni0.42Co0.16O2 has been successfully prepared by the solid state reaction method, and was characterized by XRD and SEM methods. The kinetics of the electrochemical insertion and extraction of lithium ions during the first three cycles in this material was investigated in detail by the open-circuit voltage (OCV), galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS) methods. The activation energies (Ea) of interfacial lithium ion transfer at various oxidation–reduction reactions were evaluated from the temperature-dependence of lithium ion transfer resistance. The results show that the electrochemical kinetics of the lithium ion extraction and insertion reactions in the first three cycles of this ‘composite’ material is mainly controlled by the Li2MnO3 and Li2MnO3-related components in this material. The lithium ion extraction processes from the Li2MnO3 component and LiMnO2 component (after the 1st cycle) are kinetically limited as compared with that from the LiMn0.42Ni0.42Co0.16O2 component, and the lithium ion insertion processes into the MnO2 (after the 1st cycle) component are kinetically limited as compared with that into the Mn0.42Ni0.42Co0.16O2 component. In addition, the interface reaction of the lithium ion into the Mn0.42Ni0.42Co0.16O2 component is also easier than that of the lithium ion into the MnO2 component originated from the Li2MnO3 component.

Integrating a Photocatalyst into a Hybrid Lithium–Sulfur Battery for Direct Storage of Solar Energy

Direct capture and storage of abundant but intermittent solar energy in electrical energy-storage devices such as rechargeable lithium batteries is of great importance, and could provide a promising solution to the challenges of energy shortage and environment pollution. Here we report a new prototype of a solar-driven chargeable lithium-sulfur (Li-S) battery, in which the capture and storage of solar energy was realized by oxidizing S(2-) ions to polysulfide ions in aqueous solution with a Pt-modified CdS photocatalyst. The battery can deliver a specific capacity of 792 mAh g(-1) during 2 h photocharging process with a discharge potential of around 2.53 V versus Li(+)/Li. A specific capacity of 199 mAh g(-1), reaching the level of conventional lithium-ion batteries, can be achieved within 10 min photocharging. Moreover, the charging process of the battery can proceed under natural sunlight irradiation.

A long-life lithium–sulphur battery by integrating zinc–organic framework based separator

Lithium–sulphur batteries have attracted increasing interest due to their high theoretical specific capacity, advantageous economy, and environmental friendliness. With migration of soluble lithium polysulfide (Li2Sn, 4 < n ≤ 8) in mind, we prepared one novel Zn(II) metal–organic framework (MOF) based separator for lithium–sulphur batteries. It is able to play an efficient role as an ionic sieve for the soluble polysulfide ions. More importantly, the battery with Zn(II)–MOF based separator exhibited much lower capacity decay of 0.041% per cycle at 1C over 1000 cycles.