Sovent-Free Synthesis of Lithium Intercalated Graphites Using Thermal or Mechanical Reactors

The intercalation of lithium into graphite is an important process for energy storage devices. The industrial route of intercalation is electrolysis in pre-assembled Li-ion batteries with excess Li loading in the cathode. This has a number of disadvantages: increased battery processing time, mass and volume (from needing excess Li loading), and parallel formation of a solid-electrolyte interphase (SEI). The later is difficult to control and is a major factor in the battery's overall performance and cost. An alternative is to prepare the anode with partially lithiated graphite prior to electrolyte contact. As far as we know this has not been demonstrated. Each step – lithiation of graphite followed by making slurry with binder – presents added complications with the reactive anode powder, but it offers compelling advantages such as, increase in "usable" Li, control of SEI formation, and decreased battery processing time. These advantages may also lead to battery performance and lifetime. Also, it would also allow for investigations into spontaneous formation of SEI, which may provide insight into producing an effective artificial SEI.

As reported in the literature, LiC_x has been chemically synthesized viacontact with lithium vapor and compression with lithium metal. Both of these has a flaw: graphite reaction with Li vapor is kinetically unfavored due to the low vapor pressure of Li and it is difficult to control the stoichiometry; the compression method has been shown to produce the important stages (I–IV) but it is slow, taking 24 h. Here we present two solvent-free and compression-free methods that can produce stage I or stage II lithiated graphites. The thermal method heats graphite and lithium powders to ca. 300 °C under ultrahigh vacuum (UHV) for 24 h. The mechanical method involves ball-milling the two powders together under ultra-high purity Ar(g) atmosphere for 1 h.

Both processes work with different types of graphite sources and we compare the two synthetic methods using a battery-grade mesophase graphite. The products were characterized by x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). We show that while the mechanical reactor seems to lithiate the graphite more quickly, it may affect the morphology. Hard carbon fibers were also lithiated using the mechanical reactor. We compare the stability of the LiC₆made using mesophase graphite and carbon fiber.

XRD and NMR are used to follow the delithiated of the products when in contact with a non-zero moisture environment. It was found that a surface passivation layer is formed that protects intercalated lithium. We also report open-circuit potential (OCP) and electrochemical impedance spectroscopy measurements of the products in contact with standard Li-ion battery electrolyte. These results are used to discuss the  spontaneous formation SEI at the LiC₆interface.

Figure 1: SEM images of a) stabilized lithium metal powder; b) carbon fiber, Pyrograf I; c) MGPA; d) ball-milled lithiated-MGPA. e) X-ray diffractogram of mechanically synthesized lithiated MGPA. Inset is log-scaled expansions of regions in green boxes. (o) raw graphite; (*) LiC₁₂; (†) LiC₆.

Research was supported by the Fluid Interface Reactions, Structures, and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (RLS, NJD). XPS analysis was done with support of the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division (GMV). The authors thank Applied Science, Inc. for samples of the Pyrograf I carbon fibers and Pred Materials International for the MGPA.