Wide Operating Temperature Range Electrolyte For LiFePO₄/AG Lithium Ion Battery

Abstract:As one of the core materials of lithium ion battery, electrolyte is often considered the key component to improving the temperature performance of Lithium ion batteries. The narrow operating range of the state of art systems has been identified as one of the technical barriers associated with their use in other fields. In this work, mixed salts of LiBF4-LiBOB, LiTFSI-LiBOB and cosolvent methyl butyrate (MB) were studied. The electrolytes based on mixed salts and cosolvent are systematically investigated for Li/LiFePO4and LiFePO4/AG cells, by thermogravimetric analysis (TGA), cyclic voltammetry(CV), and charge-discharge test. Conclusions as follows:(1) Thermogravimetric analysis shows that LiBF4-LiBOB and LiTFSI-LiBOB mixed salts are thermally much more stable than LiPF6. Corrosion of Al foil in the LiTFSI-based electrolytes can be suppressed successfully by the addition of LiBOB as a co-salt.The good low temperature performance and excellent high-temperature stability make the LiTFSI-LiBOB mixed salt based electrolyte a very promising electrolyte candidate for Li-ion batteries that will be used in wide-temperature applications.(2) Ionic conductivity tests show that the optimized performance is obtained when the low temperature co-solvent methyl butanoate (MB) is used, and a high discharge capacity at low temperature is thus logical, which is able to provide up to60%capacity of room temperature at-40℃. The addition of MB solvent doesn’t have big adverse impact on the high temperatue performance of LiFePO4/Li cells. So MB is benifical for improving wide temperature performance of LiFePO4/Li cells.(3) LiFePO4/AG cells containing LiTFSI-LiBOB mixed lithium salt and MB solvent electrolytes showed good performance, which have high discharge capacity at low temperature, and good cycle performance at room temperature and high temperature. In summary, LiTFSI-LiBOB based electrolyte containing MB is a very promising electrolyte candidate for LiFePO4/AG batteries that operate over a wide temperature range.