Dimensionally Stable and Fast-Charging Graphite-Silicon Planar Composite Anode for Li-Ion Batteries

Advanced Li-ion batteries (LIBs) have been developed to have high capacity density, long cycle life, and high-rate performance for portable electronics, electric vehicles (EVs), and renewable energy storage. Graphite is currently the predominant anode material for commercial LIBs because it has a low cost, low charge/discharge plateau potential, satisfactory specific capacity (372 mAh/g), and substantially high dimensional stability; its essential role in high-energy LIBs is expected to continue. Si is a potential Li-insertion anode material that has a substantially higher capacity than graphite but is susceptible to large (>300% when fully lithiated) volume expansion. The cyclic dimensional variations during charge/discharge cycles result in pulverization and electrical disconnection from the conductive paths of the Si active materials, leading to rapid capacity reduction during the cycles. In spite of a large amount of literature devoted to solving this issue in the past years, long-lasting Si anode up to now remains as a formidable challenge. On the other hand, composite anodes comprising graphite and limited amount of Si or Si oxide may be attractive transient products for advanced high-energy LIBs before viable Si-dominant anodes are realized. Moreover, improvement in the rate capability of Si-based anodes is needed for meeting the power requirements of various applications.

In this presentation, a planar graphite-silicon composite Li-ion battery anode showing substantially higher capacities than graphite, fast-charging capability, and exceptional cycle stability will be described. The Si oxide-coated graphite flake (SGF) composite anode for Li-ion batteries (LIBs) synthesized by a unique microwave-heating process of graphite flakes (GFs) in a solution made of liquid polysiloxanes as the Si-containing precursor. Microwave-heating of the GFs induces the deposition of a conformal Si-containing conformal layer on the GF surfaces, which subsequently turns into oxide-graphite-oxide sandwiched planar composite structure. The resulting SGF exhibits a reversible specific capacity of nearly 480 mAh/g, 97% capacity retention at the current density of 2.5 A/g (~5C-rate), and 95% capacity retention after 400 cycles with an average Coulombic efficiency > 99.9%. The data suggest new strategies for both designing and synthesizing high-performance anode materials for LIB applications.