## 3 Results and Discussion
Figure 4: The EDS spectra and element mapping analysis spectra of the NFCM 90 sample. Figure 3: The TEM and HRTEM images of (**a,b**) pristine and (**c,d**) NFCM 90. The electrochemical performances of pristine and NFCM 90 samples were investigated. Figure 5a shows the discharge curves of samples at 0.1 C (1 C = 278 mA h g\({}^{-1}\)). After 50 cycles, the discharge capacities of pristine and NFCM 90 samples are 129.4 and 178.2 mA h g\({}^{-1}\), respectively. The capacity retention rates for pristine and NFCM 90 samples are 76.3% (169.4 mA h g\({}^{-1}\)) and 85.2% (209.1 mA h g\({}^{-1}\)), respectively. Figure 5b shows the charge/discharge curves of the initial cycle for pristine and NFCM 90 samples at 0.1 C. The continuity of the voltage curve during the charge-discharge cycles confirms the stability of electrode material. The charge capacities of pristine and sample NFCM 90 are 195.1 and 250 mA h g\({}^{-1}\), and the discharge capacities are 169.4 and 209.1 mA h g\({}^{-1}\), respectively. The NFCM 90 sample has a higher capacity retention, higher than those of pristine samples after 15 cycles (92.3% > 90.4%) and 30 cycles (87.5% > 82.8%). Pretreatment with an appropriate amount of NH\({}_{4}\)F can prevent the cathode surface from reacting with the electrolyte. Partial substitution of oxygen in the crystal by fluorine improves the cycling stability of the NH\({}_{4}\)F-pretreated samples. The performances of lithium batteries can be degraded by treatment with an excessive amount of NH\({}_{4}\)F. Electrolyte corrosion by NH\({}_{4}\)F therefore needs to be considered. Figure 5c shows the discharge curves of pristine and NFCM 90 samples at various rates from 0.1 to 2 C. The voltage charge must be kept constant for 30 min to reach the predetermined voltage when the constant current charge at rates higher than 0.5 C. The discharge capacities of the NFCM 90 sample at 0.1, 0.5, 1, and 2 C are 206.4, 161, 106, and 47.5 mA h g\({}^{-1}\), respectively. The capacity of the NFCM 90 sample returns to 182.8 mA h g\({}^{-1}\) (88.6%) when the current rate finally returns to 0.1 C, whereas the pristine sample capacity returns to 141.6 mA h g\({}^{-1}\) (85.7%). This suggests that the NFCM 90 sample has better reversibility. Formation of spinel phase after NH\({}_{4}\)F pretreatment can improve the lithium ion diffusion rate, form oxygen vacancies to limit the release of O\({}_{2}\) from the cathode surface,  
Figure 5: **(a)** Comparative cycling performances and coulombic efficiencies at 0.1 C; **(b)** galvanostatic charge—discharge curves at 0.1 C over a potential window of 2.5–4.5 V; **(c)** comparative rate capabilities of pristine and NFCM 90 samples; **(d)** the cyclic voltammetry (CV) curves of pristine and NFCM 90 samples. and improves the structure of the solid electrolyte interphase (SEI) film on the cathode surface. These factors may be responsible for the better rate capacity of the NFCM 90 sample. Figure 5d shows the cycle voltammetry results for the pristine and NFCM 90 samples. Lithium ion extraction ability is related to oxidation peak, and lithium ion insertion ability is related to reduction peak. The specific capacity of the electrode material is related to the redox peak area. The potential interval between the anode peak and the cathode peak represents the electrode polarization capacity, which indicates the reversibility of lithium ion insertion and extraction. The potential interval for the NFCM 90 sample is 0.39 V, which is smaller than the pristine one (0.44 V), indicating a smaller electrode polarization. The redox peak for the NFCM 90 sample is larger than that for the pristine sample, which indicates a higher specific capacity. Figure 6a shows the EIS results for pristine and NFCM 90 samples. Figure 6b shows the fitted experimental results and equivalent circuits (inset).