Graphite-Based Anodes for Safe, Fast-Charging, and Long-Lasting Solid State Batteries

Solid state batteries offering fast-charging and high energy density have been extensively researched worldwide to meet the growing demand in electric vehicles, utilities, and consumer electronics. Graphite is the proven anode material in commercial batteries; it is low cost, more stable than its alternatives (e.g., silicon) with less change in volume, and easily scaled for mass production. However, the low capacity (372mAh/g) along with sluggish charging limits its application to the solid state batteries. In this study, graphite was functionalized in three ways; i) carbon coating on the surface, ii) expansion of interlayer spacing, and iii) reduction of graphite oxide in which i) and ii) were to improve the charge/discharge rate through fast diffusion of lithium ions and iii) was to further increase capacity by introducing oxygen-containing groups that accommodate lithium ions during the lithium ion intercalation/de-intercalation. The functionalized graphites were applied in a solid state cell employing a glassy oxysulfide solid electrolyte for half-cell and full-cell configurations. Figure 1A shows Raman spectra of pristine and functionalized graphites. R or ID/IG indicates the degree of disorder in the graphite structure, where ID and IG represent the intensity of G-band (stretching mode of sp2 carbon lattice, around 1580 cm-1) and D-band (defects, around 1350 cm-1), respectively. The D’-band (defects, around 1620 cm-1) overlapped with the G-band was detected in all graphite samples. R increased in the following order: carbon-coated (CCGr), expanded (ExpGr), and reduced graphite oxide (RedGO). Figure 1B shows discharge curves of the first cycles of the graphites at different discharge rates in full cells. Carbon coating and interlayer expansion improved discharge capacity and rate capability, while the reduction of graphite oxide worsened them. Carbon coating reduced the interface resistance between graphite and solid electrolyte without creating defects. For its practical application, carbon precursor, carbonization process, and the thickness of carbon coating need to be optimized. Expansion of interlayer spacing in graphite increased lithium ion diffusion which reduces the energy barrier for lithium ions to diffuse through the space between layers. It also improved kinetics for the lithium ion intercalation/de-intercalation by reducing the charge transfer resistance. Functionalization showed enhanced performance, e.g., at 0.15mA the capacity increased by 16% and 29% for CCGr and ExpGr, respectively. On the other hand, RedGO showed higher charge (lithiation) capacity than pristine and expanded graphites (not shown), but the discharge capacity was substantially diminished, e.g., 0.29mAh at 0.15mA. The oxygen-containing groups in the RedGO accommodated more lithium ions, but the de-lithiation required high overpotential. The reduction of graphite oxide leads to a multitude of defects, with the oxygen-containing functional groups significantly interacting with intercalated lithium ions. The oxygen content and functional groups in the RedGO are being investigated to enhance both the rate capability and specific capacity of the pristine graphite. In summary, the carbon coating and interlayer expansion improved the rate capability and capacity of a glassy electrolyte based solid state cell. The cells are presently undergoing evaluation for extended cycling performance. Functionalization of graphite may provide a path for safe, low cost, fast-charging, and light-weight solid state batteries. Figure 1