An Experimental Approach to Assess Fluorine Incorporation into Disordered Rock Salt Oxide Cathodes

Raynald Giovine; Eric Yoshida; Vincent C Wu; Yuefan Ji; Matthew J Crafton; Bryan D McCloskey; Raphaële J Clément

doi:10.1021/acs.chemmater.3c03138

An Experimental Approach to Assess Fluorine Incorporation into Disordered Rock Salt Oxide Cathodes

Chem Mater. 2024 Apr 3;36(8):3643-3654. doi: 10.1021/acs.chemmater.3c03138. eCollection 2024 Apr 23.

Authors

Raynald Giovine^{1

2}, Eric Yoshida^{1

2}, Vincent C Wu^{1

2}, Yuefan Ji^{1

2}, Matthew J Crafton³, Bryan D McCloskey^{3

4}, Raphaële J Clément^{1

2}

Affiliations

¹ Materials Department, University of California, Santa Barbara, Santa Barbara, California 93106, United States.
² Materials Research Laboratory, University of California, Santa Barbara, Santa Barbara, California 93106, United States.
³ Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, California 94720, United States.
⁴ Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.

Abstract

Disordered rock salt oxides (DRX) have shown great promise as high-energy-density and sustainable Li-ion cathodes. While partial substitution of oxygen for fluorine in the rock salt framework has been related to increased capacity, lower charge-discharge hysteresis, and longer cycle life, fluorination is poorly characterized and controlled. This work presents a multistep method aimed at assessing fluorine incorporation into DRX cathodes, a challenging task due to the difficulty in distinguishing oxygen from fluorine using X-ray and neutron-based techniques and the presence of partially amorphous impurities in all DRX samples. This method is applied to "Li_1.25Mn_0.25Ti_0.5O_1.75F_0.25" prepared by solid-state synthesis and reveals that the presence of LiF impurities in the sample and F content in the DRX phase is well below the target. Those results are used for compositional optimization, and a synthesis product with drastically reduced LiF content and a DRX stoichiometry close to the new target composition (Li_1.25Mn_0.225Ti_0.525O_1.85F_0.15) is obtained, demonstrating the effectiveness of the strategy. The analytical method is also applied to "Li_1.33Mn_0.33Ti_0.33O_1.33F_0.66" obtained via mechanochemical synthesis, and the results confirm that much higher fluorination levels can be achieved via ball-milling. Finally, a simple and rapid water washing procedure is developed to reduce the impurity content in as-prepared DRX samples: this procedure results in a ca. 10% increase in initial discharge capacity and a ca. 11% increase in capacity retention after 25 cycles for Li_1.25Mn_0.25Ti_0.50O_1.75F_0.25. Overall, this work establishes new analytical and material processing methods that enable the development of more robust design rules for high-energy-density DRX cathodes.