Cr/F co-doped spinel LiMn2O4 for selective lithium recovery from high-Mg salt-lake brine by hybrid capacitive deionization

Cr/F co-doped LiMn 2 O 4 was compared with undoped, Cr-only and F-only spinel controls. • Co-doping raised 1C capacity to 124.6 mAh g −1 and HCDI Li + uptake to 35.07 mg g −1 . • The co-doped electrode kept 83% of its Li + uptake over 100 cycles with 1.4% Mn loss. • A reference-cell test traced most capacity fade to the carbon counter electrode. • Li + /Mg 2+ separation factor of 24.0 in untreated old brine with acid-free desorption. Spinel LiMn 2 O 4 (LMO) is a Li + -selective intercalation host for hybrid capacitive deionization (HCDI), but Jahn–Teller distortion and Mn dissolution limit its durability in Mg-rich brines. Here, Cr 3+ and F − were introduced at the nominal cationic and anionic sites of LMO to obtain LiCr 0.05 Mn 1.95 O 3.95 F 0.05 (CFLMO), and undoped, Cr-only (CrLMO) and F-only (FLMO) controls were prepared by the same route. XRD and FTIR showed that the average spinel framework was retained, and Cr 2p and F 1s XPS detected both added elements near the CFLMO surface. At 1C, CFLMO delivered a discharge capacity of 124.6 mAh g −1 (84% of the theoretical value), compared with 92.1 mAh g −1 for LMO. In HCDI tests with 32.74 mg L −1 Li + , CFLMO reached a salt adsorption capacity (SAC) of 35.07 mg g −1 at a charge efficiency of 0.89. After 100 adsorption–desorption cycles against an activated‑carbon (AC) counter electrode, CFLMO retained 83% of its initial SAC with a Mn dissolution ratio of 1.4%; replacing the AC electrode in an LMO||AC reference cell restored part of the lost adsorption response, identifying a counter-electrode contribution to cell-level capacity loss. In untreated Yiliping salt-lake old brine (Mg 2+ /Li + mass ratio 36.1), three CFLMO electrodes recovered 37.5 ± 1.5 mg g −1 Li + with a Li + /Mg 2+ separation factor of 24.0 ± 2.7, and desorption into pure water lowered the Mg 2+ /Li + mass ratio to 1.52 ± 0.18. The matched-control and real-brine results support Cr/F-modified LMO as a candidate for selective lithium recovery from Mg-rich salt-lake brines.