LiBF4 (Lithium Tetrafluoroborate) is an alternative electrolyte salt used in non-aqueous electrolytes for lithium-ion batteries (LIBs), often as a co-salt with the dominant LiPF6 to mitigate the latter's drawbacks. Due to its high thermal stability and superior performance at high temperatures and in the presence of trace moisture, LiBF4 is strategically used in the following ways: 

High-Voltage Cathodes: LiBF4 is effective at forming a robust CEI on the positive electrode (cathode). This protection layer suppresses oxidative decomposition of the electrolyte at high operating potentials (typically >4.3 V), thereby enhancing the battery's cycle life and reducing impedance growth.

Wide/Low-Temperature Electrolytes: Its good performance at sub-zero temperatures makes it a key component in electrolytes designed for applications that require a wide operating temperature range.

Co-Salt Blends (The Synergistic Effect): LiBF4 is most often used as an additive or co-salt alongside LiPF6 (e.g. 1:1 molar ratio). This blend takes advantage of the best features of both salts: (a) LiPF6 ensures high ionic conductivity and forms the necessary SEI on the anode; (b) LiBF4 ensures thermal stability and forms the protective CEI on the cathode.

Notes: Please store the LiBF4 liquid electrolyte in the glovebox due to its sensitivity to humidity.

References: 

1. S. S. Zhang, et al. Study of LiBF4 as an Electrolyte Salt for a Li-Ion Battery, 2002,149,  A586.
2. S. Zhang, et al. Low-temperature performance of Li-ion cells with a LiBF4-based electrolyte, J. Solid State Electrochem., 2003, 7, 147-151.