Effect of Lithium Bis(oxalato) Borate (LiBOB) As an Additive in Electrolyte for Enhanced Cycling Stability of Li-Rich Li1.2Ni0.16Mn0.56Co0.08O2 cathodes

In recent years, Li and Mn rich layered  xLi₂MnO₃.(1-x)LiMO₂ (M=Mn, Co and Ni)  compounds are considered as attractive cathode materials for high energy density Li-ion batteries as these materials can provide capacities ≥ 250 mAh g^-1 [1-3]. These are considered as structurally integrated materials of two phases; Li₂MnO₃ with monoclinic (C/2m) and other LiMO₂ with rhombohedral phase. These integrated cathode materials are usually cycled to potentials higher than 4.5 V during charging in the 1^st cycle in order to activate the initially inactive Li₂MnO₃ component. The activation process involves removal of some lithium and oxygen. Once the Li₂MnO₃ phase is activated, it can undergo lithiation during discharge and contributes to the overall capacity, resulting in a high specific capacity. However, these materials suffer from capacity fading upon extensive cycling. Surface coating with a chemically inert layer of MgO [4], Al₂O₃ [5], AlF₃ [6] and AlPO₄ [7] etc. have been used to mitigate the capacity fading upon cycling of these materials.

The standard electrolyte solution EC/DMC-LiPF₆ may undergo oxidation on the cathodes surface at potential higher than 4.5 V. An alternative route is the use of additives that were found to be useful for high voltage applications by reducing the possibility of detrimental anodic reactions via formation of protective surface layers on the cathodes. LiBOB has been studied as an important additive for improving the cycling stability of high voltage cathodes based on LiNi_0.5Mn_1.5O₄[8, 9]. It is known that the oxidation of LiBOB on cathode surfaces at high potentials results in the formation of passivating surface films. Hence the presence  of LiBOB as an additive to standard solutions may have a positive impact on the electrochemical performance of Li and Mn rich layered cathode materials which are usually cycled to potentials > 4.5V vs. Li.

In the present study, Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ synthesized by self-combustion reaction (SCR) was studied as a cathode material for advanced Li ion batteries in standard electrolyte solutions with and without LiBOB at 30 and 45 ^◦C. It exhibited an initial specific capacity around 270 mAhg^-1, which stabilized at about 215 mAh  g^-1 during 50 cycles in standard electrolyte solutions. When these cathodes were tested with LiBOB as an additive in solution, a capacity retention of 98 % could be demonstrated during 50 cycles due to a unique stabilizing effect of this additive (Fig. 1). When cycled at 45 ^◦C, the capacity retention of Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ cathodes in LiBOB containing electrolyte was found to be about 97 % after 50 cycles as compared to 78 % in standard electrolyte solutions. The presence of LiBOB in solutions leads to relatively low impedance of these cathodes. HRTEM measurements show that while the initially present Li₂MnO₃ phase completely disappears upon cycling (above 4.5V) in standard electrolyte solutions, it does not fully disappear upon cycling in solutions containing LiBOB and may help in stabilizing the capacity of these integrated cathode materials. These results will be discussed.

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Fig. 1 Galvanostatic charge discharge cycles of Li_1.2Ni_0.16Mn_0.56Co_0.08O₂at C/10 rate in standard electrolyte containing (a) 0, (b) 2 and (c) 4 wt % LiBOB; (d) specific capacity vs. cycle number plot.