RESUMO
Fifty years after its introduction, the lithium-carbon monofluoride (Li-CFx) battery still has the highest cell-level specific energy demonstrated in a practical cell format. However, few studies have analyzed how the main electrochemical discharge product, LiF, evolves during the discharge and cell rest periods. To fill this gap in understanding, we investigated molecular-level and interfacial changes in CFx electrodes upon the discharge and aging of Li-CFx cells, revealing the role of LiF beyond that of a simple discharge product. We reveal that electrochemically formed LiF deposits on the surface of the CFx electrode and subsequently partially disperses into the electrolyte to form a colloidal suspension during cell aging, as determined from galvanostatic electrochemical impedance spectroscopy (EIS), solid-state 19F nuclear magnetic resonance (NMR), dynamic light scattering (DLS), and operando optical light microscopy measurements. Electrochemical LiF formation and LiF dispersion into the electrolyte are distinct competing rate processes that each affect the cell impedance differently. Using knowledge of LiF dispersion and saturation, an in-line EIS method was developed to compute the depth of discharge of CFx cells beyond coulomb counting. Solid-state 19F NMR measurements quantitatively revealed how LiF and CF moieties evolved with discharge. Covalent CF bonds react first, followed by a combination of covalent and ionic CF bonds. Quantitively correlating NMR and electrochemical measurements reveals not only how LiF formation affects cell impedance but also that CF bonds with the most ionic character remain unreacted, which limits realization of the full theoretical specific capacity of the CFx electrode. The results reveal new insights into the electrochemical discharge mechanism of Li-CFx cells and the unique role of LiF in cell discharge and aging, which suggest pretreatment strategies and methods to improve and measure the performance of Li-CFx batteries.
RESUMO
The tetracyanoethylene (TCNE) electron-transfer salts of meso-tetrakis(4-methoxyphenyl)- and meso-tetrakis(2-fluorophenyl)porphinatomanganese(II), [MnTOMePP][TCNE].2PhMe, 1, and [MnTFPP][TCNE].2PhMe, 2, respectively, have been structurally and magnetically characterized. 1 and 2 belong to the triclinicP&onemacr; (No. 2) space group [1, a = 9.896(2) Å, b = 10.256(3) Å, c = 14.447(2) Å, alpha = 82.64(2) degrees, beta = 92.40(1) degrees, gamma = 109.07(2) degrees, T = -80 degrees C, Z = 1, and R(F(o)) = 0.0295; 2, a = 10.185(5) Å, b = 11.081(3) Å, c = 12.378(3) Å, alpha = 107.55(2) degrees, beta = 82.88(3) degrees, gamma = 111.09(3) degrees, T = -80 degrees C, Z = 1, and R(F(o)) = 0.0421]. 1 and 2 are coordination polymers with the Mn(III) sites bridged by trans-&mgr;-sigma-[TCNE](*)(-). The [TCNE](*)(-) is orientationally disordered in 2, with the minor form being rotated by 90 degrees with respect to the major form and in the same plane as the major form in an approximate 5:1 ratio. The Mn-N and intrachain Mn.Mn distances are 2.289 and 2.313 Å and 10.256 and 10.185 Å, and the Mn-N-C and the dihedral angle between the mean MnN(4)-porphyrin and the mean [TCNE](*)(-) planes are 165.53 and 78.1 degrees and 148.6 and 55.4 degrees for 1 and the occupancy-weighted average of 2, respectively. For 1 above 250 K the susceptibility can be fit by the Curie-Weiss expression with theta = -65 K, while between 75 and 190 K an effective theta of +21 K is observed; for 2 above 225 K, theta is -71 K, while between 40 and 100 K the effective theta is +45 K. The initial negative theta values along with minima in the chiT(T) data at 134 K (X = OMe) and 240 K (X = F) are consistent with antiferromagnetic coupling. One chi"(T) and two chi'(T) peaks are observed for both 1 and 2. The higher temperature [ approximately 10 K (1) and 12.5 K (2)] chi'(T) peaks are frequency independent, lack a corresponding chi"(T) absorption, and are assigned to a transition to an antiferromagnetic state. In contrast, the lower-temperature frequency dependent [5.6 K (1) and 7.5 K (2) at 10 Hz] chi'(T) peaks have a corresponding frequency dependent chi"(T) peaks and are assigned to the transition to a spin-glass or superparamagnetic state. The T(c)'s are defined as chi'(T) data taken at 10 Hz and are listed above.
