Robust Lif–Rich SEI Inducing Protective Layer for High Energy Density Si Anodes in Li–Ion Batteries
Authors: Lee Seungho , Yang Subi , Lee Juhyun , Jeong Woojin , Han Seungmin , Roh Kwang Chul , Choi Junghyun 
Published: December 2024

As the electric vehicle (EV) market has rapidly grown, the need for Li ion batteries (LIBs) with high energy density is increasing to achieve a longer driving range. However, since the graphite anodes in conventional LIBs have almost reached their theoretical capacity, it is urgently needed to develop a new battery chemistry to meet the demands for long-range EVs. In this regard, Si is considered as the most promising anode materials for high-energy-density LIBs owing to its high theoretical capacity (3579 mAh g-1), low operating potential (~ 0.4 V vs. Li/Li+), and earth abundance. Unfortunately, the employment of Si anodes is hindered by the large volume change (~ 300 %) during the lithiation/delithiation process, which accompanies with electrode pulverization, unstable solid electrolyte interphase (SEI), and continuous electrolyte decomposition.
The SEI layer is the interface layer between the electrolyte and anode, which is formed via the reduction of the electrolyte components during the first lithiation process. Typically, the SEI layer is depicted as a mosaic structure that is mainly consisted of organic and inorganic constituents. The organic SEI constituents, which are the byproducts of the decomposition of the electrolytes, show poor Li ion conductivity with an unstable structure. In contrast, the inorganic SEI constituents are ideal SEI components owing to their excellent electrochemical stability. The ideal SEI layer should not only passivate the anode surface and prevent the direct contact of anode surface with the electrolyte, which causes continuous electrolyte decomposition, but also provide exhibit excellent Li ion transfer kinetics. In this respect, LiF is considered as the most ideal SEI component with its stability against the electrolyte and the high Li ion conductivity.
To achieve a LiF-rich SEI layer on the Si anode, various strategies have been devised, such as electrolyte additive and LiF coating on the anode surface. The electrolyte additives such as FEC, has been widely used to construct a LiF-rich SEI layer. Unfortunately, the additives are continually consumed over repetitive cycling, which eventually results in a rapid capacity decay of the Si anodes, when they are exhausted. In case of the LiF coating, the coating layer indeed induces LiF-rich SEI layer during the first cycle. However, if the coating layer has insufficient mechanical strength, the severe volume change during the lithiation/delithiation process leads to the breakage of the SEI layer, resulting in uncontrollable electrolyte decomposition and following (re-)formation of the SEI layer. This causes a depletion of electrochemically available Li ions in the electrolyte and a thick-insulating SEI layer with poor Li ion kinetics. Therefore, not only forming an inorganic-rich SEI layer, but also constructing a robust interfacial layer with sufficient physicochemical stability is highly important to achieve stable cycle performances of the Si anodes.
In this study, we propose a new strategy to construct a robust LiF-rich SEI layer on the Si anode by employing a LiF-rich SEI inducing protective layer (LPL) comprising of aluminum fluoride (AlF3) and poly(acrylic acid) (PAA) (LPL@Si). In the LPL@Si, the AlF3 induces a LiF-rich SEI layer with high Li ion conductivity, while the polymeric PAA provides a sufficient mechanical strength to alleviate the large volume change of the Si anode. In addition, the PAA in the LPL also acts a role of SEI stabilizer, which promotes the faster formation of the LiF-rich SEI layer owing to the abundant carboxyl groups, resulting in a rapid reduction of Li salts in the electrolyte. Due to these synergistic effects, the LPL@Si anode delivered significantly enhanced electrochemical performances in Li metal cell tests compared to that of the bare Si anode. Moreover, the LPL@Si anode exhibited an enhanced cycle performance than the bare Si anode in a full cell test with a NCM811 cathode. These results prove that our strategy for constructing a mechanically stable LiF-rich SEI layer would initiate further studies for achieving a highly stable Si anode in next-generation high energy density LIBs.