Development of nanostructured anode materials to mitigate capacity degradation for next-generation lithium-ion batteries

Abstract

Lithium-ion batteries (LIBs) are electrochemical energy storage devices widely used in the electronics and electric vehicles, while the current performance of LIBs can not meet the increasing demands of the key markets, especially for the electric vehicles. To develop next-generation LIBs, advanced nanomaterials are being studied to replace the commercial anode material, graphite owning a low theoretical capacity, 372 mAh g-1 and exhibiting sluggish kinetics. The well-developed anode materials store lithium ions via three processes, namely intercalation (graphite, titanium oxides), alloying (metals, semi-metals) and conversion (MaXb, M=metal, X=O, S, F, P, N). Materials using the three mechanisms typically suffer from slow kinetics and large volume change during lithium storage, leading to serious capacity fading at high current densities in comparison with the materials processing capacitive storage. To address the sluggish kinetics and mitigate the capacity degradation of anode materials in lithium storage, the morphologies, electronic structures and surface properties of the materials need to be developed. Herein, strategies for controlling the materials morphology, heterojunctions and electrochemical interfaces are used to improve the kinetics and stability of materials for lithium storage. Nanostructured materials in hollow, core-shell, yolk-shell and clusters were developed to buffer the volume change in lithium storage. In the chapter on controlling material morphology, a thermal vapour method was proposed to synthesize novel structures by evaporating a low-boiling point metal out of a mixed-metal nanomaterial. Cobalt (Co) and zinc (Zn) differ in boiling point by ~2000 oC, hence Co/Zn mixed materials are feasibly tested for the proposed methodology. Zn-Co mixed-ion metal-organic-frameworks (MOFs) were solvothermally prepared as precursors and their Zn was removed via pyrolysis to leave Co3O4 nanostructures. The formed nanostructures were distinct as a result of the MOFs containing tunable amounts of Zn and Co ions. Hollow Co3O4 spheres exhibited a specific capacity of 890 mAh g-1 at 0.1 A g-1 and maintained a similar value at a large rate of 1 A g-1 after 120 cycles, indicating outstanding rate and cycling performance. Co3O4 in shapes of dumbbells, spheres and grapes were composed of clustered nanoparticles by pyrolysis. Among them, Co3O4 grapes performed the best with a high specific capacity of 861 and 606 mAh g-1 at 0.1 and 10 A g-1 respectively. Nanostructured Co3O4 displayed considerable capacity retention during cycling and at large rates by a conversion storage. Storage via alloying is another kind of process leading to high capacity but usually suffers from large volume changes and capacity degradation. Tin (Sn) is a promising alloying-type anode material for lithium storage because of the huge capacity and high conductivity. Yolk-shell and core-shell Sn@C spheres interconnected by carbon nanofibers were formed using electrospun ZnSn(OH)6@polyacrylonitrile fibers as precursors through thermal vapour pyrolysis. The developed structures exhibited a capacity retention of 91.8% after 1000 cycles at a high rate of 1 A g-1. The synthesized materials by thermal vapor method displayed large mitigation in capacity degradation compared with the relevant publications, resulting from the advanced nanostructures that can suppress volume changes in lithium storage. Heterojunctions fabricated between the metal oxides of different electronic structures benefit the kinetics of electron conduction because of the built-in electric field, boosting materials high-rate performance for lithium storage. In the chapter on material heterojunction development, ZnO, SnO2 and TiO2 were considered as these are earth abundant materials. TiO2 has many polymorphs and in the formation of a stable structure, it is feasible to tune the phase composition, thus the heterojunctions during synthesis. The tunable characteristic of TiO2 heterojunctions indicates TiO2 is an appropriate model sample to study how heterojunctions influence the Li-storage properties of metal oxides. Dandelion-shape TiO2 comprising TiO2(B) fibrils and anatase pappi displayed high kinetics for lithium storage through the enhanced interfacial charge storage process. TiO2 spheres of mixed anatase/rutile maintained a high specific capacity of 95 mAh g-1 at an ultra-high rate of 10 A g-1 (~60 C) compared with the low-rate capacities, resulting from a synergistic storage between the two phases with the anatase incorporating lithium ions more readily and rutile conducting lithium ions faster. Although the kinetics of TiO2 in lithium storage has been enhanced through heterojunction fabrication, TiO2 has a low specific capacity in comparison with other conventional-type anode materials. ZnO and SnO2 offer much higher theoretical capacities. To obtain heterojunctions using ZnO and SnO2, a complex metal oxide, Zn2SnO4 was formed and spatially confined ZnO and SnO2 generated in the composite. ZnO/Zn2SnO4/SnO2 composite displayed enhanced internal electric field arising from the electron trapping in SnO2 and facilitated kinetics. The stepwise reaction of the composite with lithium improved the structural stability via limiting the volume changes. A specific capacity of 121 mAh g-1 at 20 A g-1 and a high retention of 76.2% at 10 A g-1 after 1000 cycles were yielded. The kinetic improvement through heterojunction engineering facilitates the lithium storage and benefits the materials cycling stability. Electrochemical interface is crucial for the charge transfer and material stability in lithium storage. In the chapter concerned with interface tuning, strategies are through modifying the materials’ electronic properties, chemical interactions and surface properties. The interface structure on graphite in LIBs has been widely studied but how the materials electronic properties influence the interface is still not clear. Graphene has distinct electronic properties compared to graphite and is proposed to be a high-capacity anode material for lithium storage. Tuning the layers number of graphene layers results in a change of the material’s electronic properties and influences the interface structure. Studies of multilayer, bilayer and monolayer graphene showed that the former had a low intercalation energy and weak reduction capability to form a uniform interphase, indicating multilayer graphene is safer for lithium storage devices. Graphene is a conductive network in forming composites and a base for doping with heteroatoms, enabling graphene to interact with other materials through the surface groups. Phosphorus (P) doping induced a polar surface in P-doped graphene oxide (PGO). The surface groups of PGO were evidenced to immobilize the heterostructured SnS2/SnO2 by forming chemical interaction, leading to improved capacity retention for lithium storage from 30.3% for SnS2/SnO2 to 49.5% for PGO/SnS2/SnO2 in 500 cycles. Surface coating is a popular method to change the interface of materials for lithium storage. ZnO has a high capacity but suffers from the large volume change in lithium storage but ZnS displays much less volume change. Sulfidation of the ZnO surface by forming a ZnS surface layer is a strategy to mitigate the volume change of the materials and suppress the capacity degradation in lithium storage. Sulfidation of the MOF-pyrolyzed ZnO altered the surface to be ZnS, resulting in an increase in the specific capacity by 38% and mitigated capacity degradation during cycling compared with ZnO. Nanoengineering of the electrochemical interface of materials can stabilize materials’ nanostructures during lithium storage and lead to a stable storage performance.