Profiling of Anode Surface Cycled in LiBOB-based Electrolyte of Li Ion Batteries
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Profiling of Anode Surface Cycled in LiBOB-based Electrolyte of Li Ion Batteries Unchul Lee, Kang Xu, Sheng S. Zhang, and T. Richard Jow Sensor and Electronic Devices Directorate U. S. Army Research Laboratory, Adelphi, MD 20783-1197 Abstract As the youngest battery chemistry, Li ion technology was made possible by the formation of stable electrode/electrolyte interfaces. The correlation between the electrochemistry and the surface profile of the graphitic anode was studied in this work with a new salt lithium bis (oxalate) borate (LiBOB). In an attempt to depict a dynamic picture of the formation of graphite/electrolyte interface during the initial forming cycle, we employed X-ray photoelectron spectroscopy in combination with the “pre-formation” technique to establish the dependence of the surface chemistry on the forming potential of the anode. A progressive transition in the 1s electron binding energies of the major elements was observed as the lithiation proceeds; however, the surface chemical species as well as their abundances seemed to stabilize around 0.55 V and remained constant during the subsequent delithiation process, indicating that a stable solid electrolyte interface (SEI) exists thereafter. Integrating the information revealed by different analyses, we believe that the reductive decomposition of BOB--anion starts at ca. 1.00 V, while the effective protection of graphene surface by SEI is available after the anode is lithiated below the potential of 0.55 V. Introduction In the wake of developing lithium bis(oxalate)borate (LiBOB) as a potential replacement for lithium hexafluorophosphate (LiPF6) in lithium ion devices, 1~2 extensive electrochemical and spectroscopic characterizations have been carried out on this new salt.3~6 The initial successes of this salt, believed to originate from its unique surface chemistry on graphitic anode, 5, 6 have drawn an intense interest in the battery research community 7~13. According to the knowledge accumulated so far on this new salt, LiBOB-electrolytes have demonstrated pronounced improvements of stability in lithium ion environment at elevated temperatures where different anode (graphite, MCMB) and cathode (LiNixM13, 7a, 9, 12a , and its ability to stabilize the graphitic xO2, LiMn2O4) materials were used anodes in propylene carbonate (PC) confirmed that the anion is actively involved in the formation of a protective graphite/electrolyte interface 4, 5, 12b. While the anodic stability of LiBOB-based electrolytes has been observed at potentials up to 4.50 V vs. Li+/Li on various cathode surfaces 8b, 8d 12b, the thermal reactivity of LiBOB with most cathode materials — with the exception of lithium iron phosphates — under adiabatic conditions did raise concern 8c, e, the source of which might be the fact that LiBOB is one of those “too stable” salts that would fail to pre-passivate cathode surfaces (as LiPF6 does) before the thermally-activated decompositions occur 14. Since the thermal behavior of a full lithium ion device is basically dictated by the
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