Comparison of the gravimetric energy densities for various full-cells. LMO, LCO, LFP, NMC, and NCA indicate LiMn2O4; LiCoO2; LiFePO4; Li- [Co⅓Ni⅓Mn⅓]O2; and Li[Ni0.8Co0.15Al0.05]O2, respectively. Credit: ACS, Kim et al. Click to enlarge.

Although silicon is a very attractive anode material because of its higher energy density, its huge volume change over repeated charge−discharge cycles impairs the cycle life through pulverization of active particles, film delamination, and unstable solid−electrolyte interphase (SEI) formation.

As an alternative approach to avoid the known drawbacks, silicon monoxide (SiO_x, x ≈ 1) phase has been recently adopted because its SiO_y (y near 2) background matrix can buffer the volume expansion of inner Si nanodomains. On the basis of this structural advantage, some SiO_x electrodes demonstrated very stable cycling performance.

Nonetheless, most of these phases suffer from inferior performance in a crucial parameter, namely initial Coulombic efficiency (ICE). This shortcoming originates primarily from Li ion trapping in the matrix and SEI layer formation during the first lithiation. Hence, in an overall performance viewpoint, while facilitating the long-term cycling performance, the background matrix, in turn, sacrifices the ICE. The compromised ICE would impose a great hurdle in constructing full-cells in practical applications because poor ICE necessitates an excess amount of cathode active material solely for the first cycle, leading to a lessened total energy density. For reference, current commercial LIBs involving graphite anodes usually have ICEs higher than 85% at low C-rates (∼0.1C).

In an attempt to catch these two challenging rabbits (cycle life and ICE), the current study has developed delicately controlled prelithiation for SiO_x anodes.

—Kim et al.

Attaining the optimal degree of lithiation is very critical for stable full-cell operations, the team noted. Insufficient lithiation leaves Li trapping sites and does not improve the ICE enough, but overlithiation removes the possibility of accepting Li ions during the actual alloying reaction in the first charge.

To hit the sweet spot, the team prelithiated the pristine electrode via an electrical short with Li metal foil in the presence of an optimized circuit resistance while simultaneously monitoring the voltage between both electrodes.

(a) Graphical illustration of prelithiation process of c-SiOx electrode and (b) its scalable roll-to-roll process scheme. Credit: ACS, Kim et al. Click to enlarge.

The procedure is compatible with the existing roll-to-roll manufacturing line and could be immediately applicable to the current state-of-the-art LIB anodes containing SiO_x, they concluded.

Resources

• Hye Jin Kim, Sunghun Choi, Seung Jong Lee, Myung Won Seo, Jae Goo Lee, Erhan Deniz, Yong Ju Lee, Eun Kyung Kim, and Jang Wook Choi (2015) “Controlled Prelithiation of Silicon Monoxide for High Performance Lithium-Ion Rechargeable Full Cells” Nano Letters doi: XXX10.1021/acs.nanolett.5b03776XX