Salt Effects on SEI Formation in Graphite and LiNi0.5Mn1.5O4 Based Half Cells

Lithium orthoborate salts have been intensively studied during the last two decades because of their distinct thermal stability and their potential of replacing commercial LiPF₆, which has low chemical and thermal stability.^1-3 One particular member is lithium bis(oxalato) borate (LiBOB), which showed significantly improved thermal stability over LiPF₆ at 70 ^oC. ⁴  Also a claimed unique feature of LiBOB was the participation in the solid electrolyte interphase (SEI) formation by the BOB^- anion, which allowed the use of pure propylene carbonate (PC) based electrolyte in graphite electrode based cells without causing solvent co-intercalation and graphite exfoliation. ⁵

Herein, we report the synthesis and characterization of a new LiBOB analog salt, lithium bis(2-methyl-2-fluoromalonato)borate (LiBMFMB), ⁶ which is more stable than the previously reported lithium borate salt, lithium bis(fluoromalonato)borate (LiBFMB). ^{7, 8} Fig. 1 shows the initial cyclic voltammetry of LiBMFMB, LiPF₆ and LiBOB based electrolytes at a scan rate of 0.1mV/s. Clearly, the reduction processes are dramatically different because of the salt difference. Generally, each salt shows distinct reduction feature in each of the following three voltage regions, that is, I (>1.5 V), II (1.0-1.5V) and III (0.5- 1.0V) ( inset). For example, the conspicuous reduction peak at 1.75 V due to the reduction of the BOB anion is dominate in region I, the reduction of the LiBMFMB based electrolyte is dominant in region II, and the reduction of the LiPF₆ based electrolyte is dominant in region III.  The CVs in Fig. 1 suggest that both LiBOB and LiBMFMB based electrolytes are reduced mainly on the surface of the graphite electrode, although one difference is that the reduction of the BOB anion is dominant for the former while the reduction of the carbonate solvents is domninant for the latter. For the LiPF₆ based electrolyte, the redcution is mainly happened after being co-intercalated with Li⁺ into the graphite layers.

Fig. 2a shows the charge-discharge profile of the LiNi_0.5Mn_1.5O₄||Li half-cell based on 0.8 M LiBMFMB/EC-EMC (1/2, by wt.) under  different current rates at room temperature. The cell exhibits good cycling stability, for example, the reversible capacities after 5 cycles at C/10, 40 cycles at C/5, and 50 cycles at C/2 are 111.5, 93.9 and 84.0 mAh g^-1, respectively. The above cycling performance is much better than the half-cells based on 0.5 M LiBFMB/EC-DMC-DEC and 1.0 M LiBFMB/PC. ⁷ This is attributed to the stable structure of the LiBMFMB salt, which results in less electrolyte decomposition and thinner SEI layer. In addition to good cycling performance in the LiNi_0.5Mn_1.5O₄||Li half-cell, the new salt electrolyte also exhibits good performance in natural graphite (NG) based half-cell. As shown in Fig. 2b, the reversible capacities after 5 cycles at C/10, and 90 cycles at C/5 are 324.3 and 230.5 mAh g^-1, respectively.

 During this talk the salt effect, such as LiBOB, LiPF₆, LiBMFMB, as well as different chain length of the alkyl substituents in LiBFMB, on the SEI formation in both graphite and LiNi_0.5Mn_1.5O₄ based half-cells will be compared. In addition to cycling performance, impedance analysis, electrode surface will be analyzed by XPS and SEM.

 This research was supported by the U.S. Department of Energy’s Office of Science, Basic Energy Science, Materials Sciences and Engineering Division.

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